Calculate the first row of the score matrix during phase 2

Rework of 656963e0. Makes --ansi processing around 20% faster on plain
strings without ANSI codes.

This reverts commit 656963e0.

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.

https://github.com/junegunn/fzf/issues/444#issuecomment-321719604

When $ is the leading character in a query, it's probably not meant to
be an anchor.

https://github.com/junegunn/fzf/issues/444#issuecomment-321432803

Close #444

One can escape meta characters in extended-search mode with backslashes.

  Prefixes:
    \'
    \!
    \^

  Suffix:
    \$

  Term separator:
    \<SPACE>

To keep things simple, we are not going to support escaping of escaped
sequences (e.g. \\') for matching them literally.

Since this is a breaking change, we will bump the minor version.

Close #444

It is reported that it can have an unwanted side effect of clearing the
screen on terminal emulators that do not properly support it.

Patch suggested by @arya.

Close #1011

Manually inline function calls in a tight loop as Go compiler does not
inline non-leaf functions. It is observed that this unpleasant code
change resulted up to 10% performance improvement.

Close #995

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.

- Fix incorrect display of the last line when more than a line is
  wrapped above
- Avoid unnecessary flickering of the window