Support standalone files resolving links, refactoring, and more from a wiki

[chipsenkbeil]

When loading standalone files, a common usecase is to have these files mirror those of files in a wiki. This is how vim is able to get information about an unsaved buffer to properly select elements and do other actions.

Having some way to associate a standalone vimwiki file with a wiki directory during link resolution, refactoring, and more is an important feature.

[chipsenkbeil]

This process is referred to creating a temporary wiki.

For vimwiki, it creates a temporary wiki to append to the vimwiki list. It allows:

1. wiki-linking to other files in same directory or subdirectories
2. highlighting wiki pages
3. HTML generation

The temporary wiki uses all of the vimwiki-local-options for the new wiki except three:

1. Path is set using file's location
2. Extension is set using file's extension
3. Syntax is default (vimwiki) unless vimwiki-register-extension is configured for the current file's extension