Instead of notifying the event coordinator (EventBox) whenever a new
line is arrived, start a background goroutine that periodically does the
task. Atomic.StoreInt32 is much cheaper than mutex synchronization
that happens during EventBox update.

I was too quick to release 0.16.9, this commit makes --ansi processing
even faster.

When --with-nth is used, fzf used to preprocess each line and store the
result as rune array, which was wasteful if the line only contains ascii
characters.

With zero result.

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

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

Close #790

Related: #660

- Make structs smaller
- Introduce Result struct and use it to represent matched items instead of
  reusing Item struct for that purpose
- Avoid unnecessary memory allocation
- Avoid growing slice from the initial capacity
- Code cleanup

https://golang.org/doc/go1.5

In the best case (all ascii), this reduces the memory footprint by 60%
and the response time by 15% to 20%. In the worst case (every line has
non-ascii characters), 3 to 4% overhead is observed.

Close #598

This commit compensates for the performance overhead from the
extended tiebreak option.

Close #400

and allow using --header and --header-lines at the same time.

Close #346.

A la grep. Close #345

    > time cat /tmp/list | fzf-0.10.1-darwin_amd64 --ansi -fqwerty > /dev/null

    real    0m4.364s
    user    0m8.231s
    sys     0m0.820s

    > time cat /tmp/list | fzf --ansi -fqwerty > /dev/null

    real    0m4.624s
    user    0m5.755s
    sys     0m0.732s

    > wc -l /tmp/list2
     2594098 /tmp/list2

    > time cat /tmp/list2 | fzf-0.10.1-darwin_amd64 -fqwerty > /dev/null

    real    0m5.418s
    user    0m10.990s
    sys     0m1.302s

    > time cat /tmp/list2 | fzf-head -fqwerty > /dev/null

    real    0m4.862s
    user    0m6.619s
    sys     0m0.982s

- dark:  the current default for 256-color terminal
- light: color scheme for 256-color terminal with light background
- 16:    the default color scheme for 16-color terminal (`+2`)
- bw:    no colors (`+c`)