Add support for completion inside paired delimiters

It seems that language server completions are not suggested for snippets inside paired delimiters. This makes it difficult to use ALE in conjunction with a delimiter auto-pairing plugin (or when delimiters are manually paired). This does not appear to be a limitation of the language servers, as other LSP clients (e.g., LanguageClient-neovim) can make suggestions inside paired delimiters.

Working examples for golang and python.

Examples of no completion suggestions when typing inside paired function delimeters:

This is with the following ALE config:

```vim
Current Filetype: python
Available Linters: ['flake8', 'mypy', 'prospector', 'pycodestyle', 'pyflakes', 'pylint', 'pyls', 'pyre', 'vulture']
Enabled Linters: ['flake8', 'pyls']
Suggested Fixers:
'add_blank_lines_for_python_control_statements' - Add blank lines before control statements.
'autopep8' - Fix PEP8 issues with autopep8.
'black' - Fix PEP8 issues with black.
'isort' - Sort Python imports with isort.
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
'yapf' - Fix Python files with yapf.
Linter Variables:
let g:ale_python_flake8_auto_pipenv = 0
let g:ale_python_flake8_change_directory = 1
let g:ale_python_flake8_executable = 'flake8'
let g:ale_python_flake8_options = ''
let g:ale_python_flake8_use_global = 0
let g:ale_python_pyls_auto_pipenv = 0
let g:ale_python_pyls_executable = 'pyls'
let g:ale_python_pyls_use_global = 0
Global Variables:
let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = v:null
let g:ale_command_wrapper = ''
let g:ale_completion_delay = 100
let g:ale_completion_enabled = 1
let g:ale_completion_max_suggestions = 50
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '[%linter%]% code%: %s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 1
let g:ale_fixers = {'*': ['remove_trailing_lines', 'trim_whitespace'], 'python': ['black']}
let g:ale_history_enabled = 1
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = v:null
let g:ale_lint_delay = 200
let g:ale_lint_on_enter = 0
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 0
let g:ale_lint_on_save = 0
let g:ale_lint_on_text_changed = 'never'
let g:ale_linter_aliases = {}
let g:ale_linters = {'go': ['golangserver'], 'python': ['pyls', 'flake8']}
let g:ale_linters_explicit = 0
let g:ale_list_vertical = v:null
let g:ale_list_window_size = v:null
let g:ale_loclist_msg_format = v:null
let g:ale_max_buffer_history_size = 20
let g:ale_max_signs = v:null
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 1
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = v:null
let g:ale_sign_error = '✖'
let g:ale_sign_info = v:null
let g:ale_sign_offset = v:null
let g:ale_sign_style_error = v:null
let g:ale_sign_style_warning = v:null
let g:ale_sign_warning = '⚠'
let g:ale_statusline_format = v:null
let g:ale_type_map = v:null
let g:ale_use_global_executables = v:null
let g:ale_virtualtext_cursor = 0
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
Command History:
(started) ['/bin/zsh', '-c', '''pyls''']
```

Moreover, the minimal RC file used is:

```vim
if has('nvim')
let vimplug = "$XDG_DATA_HOME/nvim/site/autoload/plug.vim"
else
let vimplug = "~/.vim/autoload/plug.vim"
endif

let plugins = "$XDG_DATA_HOME/nvim/site/plugins"

let g:ale_completion_enabled = 1

if empty(glob(vimplug))
execute ' !curl -fLo ' . vimplug . ' --create-dirs ' .
\ 'https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
autocmd VimEnter * PlugInstall --sync
endif

call plug#begin(plugins)

Plug 'w0rp/ale'

call plug#end()

let g:ale_linters = {
\ 'python': ['pyls', 'flake8'],
\ 'go': ['golangserver'],
\ }
let g:ale_fixers= {
\ '*': ['remove_trailing_lines', 'trim_whitespace'],
\ 'python': ['black'],
\ }
let g:ale_fix_on_save = 1
let g:ale_completion_enabled = 1
let g:ale_completion_delay = 100
let g:ale_echo_msg_format = '[%linter%]% code%: %s'
let g:ale_lint_on_enter = 0
let g:ale_lint_on_save = 0
let g:ale_lint_on_text_changed = 'never'
let g:ale_open_list = 1
let g:ale_set_ballons = 1

" set completeopt=longest,menuone,preview,noselect
```