Enable configuration of the function used to retrieve the buffer list

[s1341]

It's useful to enable configurability of the function used to retrieve the buffer list, so that certain buffers can be filtered, for example.

This pull request adds a new global variable which may contain the name of the function which provides the buffer list, allowing for extensibility of fzf's buffer related commands.

[vasilakisfil]

yes that's super helpful, any chance it gets accepted?

[s1341]

Seeing as I have not received any communication from the maintainer, I think it unlikely. Still useful.

[vasilakisfil]

This would also solve #393. @junegunn any thoughts ?

[krkhan]

+1, I'm currently using this as a custom patch on top of master. @junegunn any concerns with merging this one?

[s1341]

Any chance we could merge this?

[bit101]

I've been patching my fzf install to add this functionality for over a year now. Would love to see this merged if at all possible.