Fix https://github.com/junegunn/fzf.vim/issues/404

Thanks to @janlazo.

By not storing item index twice, we can cut down the size of Result
struct and now it makes more sense to store and pass Results by values.
Benchmarks show no degradation of performance by additional pointer
indirection for looking up index.

- Remove multiple mutex locks in partial cache lookup
- Simplify return values

Close #966

Related: #960 (relative filepaths)

Related #960, #552

With zero result.

Related: https://github.com/junegunn/fzf.vim/issues/22#issuecomment-311869805

This also modifies <C-t> behaviour.
The longest file path in the input is used as root directory for `find`
command. The remainder of the input is passed to fzf's --query as a
initial search parameters.

Fix found by @amosbird at https://github.com/junegunn/fzf/issues/953#issuecomment-310309055

closes #440

See #944

fzf should immediately continue consuming the buffer after discarding
bracketed paste mode sequence.

Close #951

If "." is given as the argument to begin <C-t> completion, the leading
"." is not correctly removed. In general, if user selects a fzf
completion, the current token should be "consumed".

Close #943

See https://github.com/junegunn/fzf/pull/933#discussion_r120011934

Close https://github.com/junegunn/fzf.vim/pull/386

Also change the example binding for `unix-word-rubout` to *ctrl-w* instead of *ctrl-u*

Close #935

- Update install script to download Windows binary if $TERM == cygwin
- Unset TERM if $TERM == cygwin (#933)
- Always use cmd.exe instead of $SHELL when running commands

Make sure to consistently calculate tiebreak scores based on the
original line.

This change may not be preferable if you filter aligned tabular input on
a subset of columns using --nth. However, if we calculate length
tiebreak only on the matched components instead of the entire line, the
result can be very confusing when multiple --nth components are
specified, so let's keep it simple and consistent.

Close #926

Related: https://github.com/junegunn/fzf.vim/issues/378