Resolves #62 - Change the way visual mode is restored after a command

[SidOfc]

This patch fixes #62. (And also mapping bug: #64)

Prior to this change, the setting g:mkdx#settings.restore_visual could not be changed at runtime, one had to restart Vim due to the way that visual mode was restored (applying gv after plug mapping).

To fix this extra <Plug> mappings were added and some existing ones were changed. Backwards compatibility is kept by keeping a copy of the original <Plug> mappings around, so they will still work if you created a custom mapping with them, just note that in visual mode, the selection will no longer be restored.

This is the list of plugs that are deprecated:

" backwards compat <= 1.8.0
noremap         <silent> <Plug>(mkdx-checkbox-next)      :call      mkdx#ToggleCheckboxState()<Cr>
noremap         <silent> <Plug>(mkdx-checkbox-prev)      :call      mkdx#ToggleCheckboxState(1)<Cr>
noremap         <silent> <Plug>(mkdx-toggle-quote)       :call      mkdx#ToggleQuote()<Cr>
noremap         <silent> <Plug>(mkdx-toggle-checkbox)    :call      mkdx#ToggleCheckboxTask()<Cr>
noremap         <silent> <Plug>(mkdx-toggle-checklist)   :call      mkdx#ToggleChecklist()<Cr>
noremap         <silent> <Plug>(mkdx-toggle-list)        :call      mkdx#ToggleList()<Cr>

And here a list of what replaced what:

• <Plug>(mkdx-checkbox-next) replaced by
  • <Plug>(mkdx-checkbox-next-n)
  • <Plug>(mkdx-checkbox-next-v)
• <Plug>(mkdx-checkbox-prev) replaced by
  • <Plug>(mkdx-checkbox-prev-n)
  • <Plug>(mkdx-checkbox-prev-v)
• <Plug>(mkdx-toggle-quote) replaced by
  • <Plug>(mkdx-toggle-quote-n)
  • <Plug>(mkdx-toggle-quote-v)
• <Plug>(mkdx-toggle-checkbox) replaced by
  • <Plug>(mkdx-toggle-checkbox-n)
  • <Plug>(mkdx-toggle-checkbox-v)
• <Plug>(mkdx-toggle-checklist) replaced by
  • <Plug>(mkdx-toggle-checklist-n)
  • <Plug>(mkdx-toggle-checklist-v)
• <Plug>(mkdx-toggle-list) replaced by
  • <Plug>(mkdx-toggle-list-n)
  • <Plug>(mkdx-toggle-list-v)

All the docs have been updated to reflect the new -{n|v} style for these plugs.