Add support for sharkdp/fd

[lithammer]

https://github.com/sharkdp/fd is the new kid on the block as an alternative to find(1), but obviously much faster (see benchmarks in README).

It would be nice to extend ListFilesCommand to check fo fd either between git and rg, or as first hit since it has some git filtering built in.

[srstevenson]

Thanks for the suggestion @Renstrom, I hadn't come across fd before! I'm wary of a proliferation of conditional branches based on what tools are installed, so instead of adding fd I've opened #20 which makes the choice of file listing tool user configurable. To use fd, you can set:

let g:picker_find_executable = 'fd'
let g:picker_find_flags = '--color=never'

I'm going to use this configuration for a bit and decide whether to switch the default tool from rg to fd, as fd looks ideally suited.

[lithammer]

This is great, thanks!

[srstevenson]

@Renstrom As of 6e93ac3, fd is now used instead of riprgrep by default.