Use g:fzf_tags_command in BTags

[villainy]

Resolves #196

Also updates docs to clarify this will only work with exuberant ctags (or a derivative like universal ctags). The existing hard coded arguments are unlikely with other tag generators.

[PezCoder]

Thank you so much for the PR. ❤️ I tested this PR against my universal ctags tags installation with the following configuration:

let g:fzf_tags_command = 'uctags -R'

The buffer tags which were empty started populating, worked like a charm !

@junegunn Can you please help review & merge this pull request if it looks good to you? 🙏

[flwyd]

Is anything blocking this pull request from being merged?

[aeciolevy]

I tried but it did not work for me. I have changed the autoload/fzf/vim.vim Should I do anything more besides changing the file?

[rbpatt2019]

Bump! I'm running into this as well - is there anything I can do to help here?