feat: add toggle_mark

[yehuohan]

Add toggle_mark command to toggle inputed mark at current line like what kshenoy/vim-signature (it uses g:SignatureMap.Leader to toggle inputed mark) does.

[chentoast]

Can you please describe really quickly the intended usecase for this; ie, what would this allow that a regular toggle could not? Thanks for the PR.

[yehuohan]

Can you please describe really quickly the intended usecase for this; ie, what would this allow that a regular toggle could not? Thanks for the PR.

Sure. This PR add the toggle_mark, which means the toggle command need a inputed mark to perform its functionality. Under the setup with require('marks').setup{mappings = { toggle_mark = 'm'}} as an example: When press 'ma':

• If there is no mark a on current line, it will add a mark a on the current cursor position.
• If there is a mark a on current line, it will remove the mark a.

[kamack38]

@chentoast Would it be possible to merge this?

[goetzc]

This would be great to have! Makes total sense, specially after having used this feature with kshenoy/vim-signature.