IndexError on aliases with no parameters

Config:
```
[alias]
s = list statuses
```

Command: `redmine s`

Error:
```
Traceback (most recent call last):
File "/home/egegunes/Dev/redmine-cli/venv/bin/redmine", line 11, in
load_entry_point('redmine-cli', 'console_scripts', 'redmine')()
File "/home/egegunes/Dev/redmine-cli/venv/lib64/python3.6/site-packages/click/core.py", line 764, in __call__
return self.main(*args, **kwargs)
File "/home/egegunes/Dev/redmine-cli/venv/lib64/python3.6/site-packages/click/core.py", line 717, in main
rv = self.invoke(ctx)
File "/home/egegunes/Dev/redmine-cli/venv/lib64/python3.6/site-packages/click/core.py", line 1132, in invoke
cmd_name, cmd, args = self.resolve_command(ctx, args)
File "/home/egegunes/Dev/redmine-cli/venv/lib64/python3.6/site-packages/click/core.py", line 1171, in resolve_command
cmd = self.get_command(ctx, cmd_name)
File "/home/egegunes/Dev/redmine-cli/redmine/cli/alias.py", line 26, in get_command
params = self.group_params(actual_cmd[1:])
File "/home/egegunes/Dev/redmine-cli/redmine/cli/alias.py", line 11, in group_params
grouped_params.append((params[i].lstrip("-"), params[i + 1]))
IndexError: list index out of range
```