Take two on the ToC changes

[benlubas]

I fixed the bug that I found in the last PR that still got merged despite the bug :P

Also in this PR:

• fixed_width option for the ToC window
• create_vsplit refactor/update to make use of the new nvim_open_win functionality in 0.10.0
• Fixed logic for computing the ToC width
• You can configure the toc command to just open but not enter the window

[benlubas]

Question, should updating the ToC also update it's width? If I open a document with no headers, it will create an empty ToC that's not too wide, then I type a long heading, the ToC remains at it's earlier width, and is not updated.

I think it would be nice, but could get annoying depending on the other settings you have to see the window keep resizing

[vhyrro]

Perhaps it would be nice to have a minimum TOC width as well? I don't think it's too jarring to have the TOC resize personally.