Add show_source option to show the source buffer

[bobrippling]

This adds a show_source option, which, if given, will return the buffer name of a given match, to allow it to be shown to the user when completing.

The output looks like:

[jekas-obps]

@bobrippling, I like your idea, but I can't reproduce your setup. Would you mind to provide a reference on how to access exposed buffer details in nvim-cmp config?

[jekas-obps]

@bobrippling, I like your idea, but I can't reproduce your setup. Would you mind to provide a reference on how to access exposed buffer details in nvim-cmp config?

@bobrippling never mind, I put show_source in wrong place. Now I see how it works, it uses same window that is used for documentation, pretty neat!

I wish that to be included in official plugin.