Support fzf search syntax

[liuchengxu]

This PR implements most of the fzf-like search syntax. Happy to have some Rust suggestions if you happen to get some time :P @ImmemorConsultrixContrarie .

TODO:

• [ ] OR operator. I decide this won't be implemented as you can use multiple exact matches instead.

Close #492

[fannheyward]

@liuchengxu I've rebuild the maple bin, but still can't search with the syntax:

Fixed with let g:clap_builtin_fuzzy_filter_threshold = 0.

[zhuzhzh]

can we support fzf-like search by default? now it needs let g:clap_builtin_fuzzy_filter_threshold = 0.

[liuchengxu]

let g:clap_builtin_fuzzy_filter_threshold = 0 now is required to ensure the search is using the Rust backend instead of the other implementations because the fzf search syntax support is only available when using the Rust binary. Perhaps we can consider making it a default behavior if the Rust executable exists.