Related #642

Fix #520
Fix junegunn/fzf.vim#42

fzf#wrap([name string,] [opts dict,] [fullscreen boolean])

- 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

By only using leaf functions

Sort performance increases as the size of each sublist decreases (n in
nlog(n) decreases). Merger is then responsible for merging the sorted
lists in order, and since in most cases we are only interesed in the
matches in the first page on the screen so the overhead in the process
is negligible.

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

Approx. 50% less memory footprint and 40% improvement in query time

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.

Fix #636. Patch suggested by @edi9999.

Fix #183

Close junegunn/fzf.vim#173 and #630

Remove `name` option from `termopen`.

`termopen` no longer accepts a `name` option, instead we should suffix the
command with `;#NAME`.

Patch suggested by Mariusz Atamańczuk

Close #627

To make it consistent with the other commands in fzf.vim

Close #617

Close #607

Close #622

Close #620