Add support for doctoc

**Name:** doctoc
**URL:** https://github.com/thlorenz/doctoc

This might be an interesting use-case, but `doctoc` is a table of content (TOC) generator for markdown files. It would be interesting to add as a markdown fixer such that `:ALEFix` would generate or update existing TOC previously generated by `doctoc`.

I am interested to hear from other users, but a fairly common markdown workflow for me is:

1. Write docs in markdown
2. Ale shows linting information/warning/errors via `markdownlint-cli`
3. Ale fixes markdown (via `prettier` or any other markdown formatter) upon save or with `:ALEFix` command
4. Run `:!doctoc --update-only %`

It would be a great experience if ALE can be configured to fix markdown like:

```viml
let b:ale_fixers = ['prettier', 'doctoc']
" or
let g:ale_fixers = { 'markdown': ['prettier', 'doctoc']}
```

And have TOC get generated or updated on save or using `:ALEFix`.