Add finer grained control over signs and highlights

I wanted to be able to customize signify in a way that wasn't possible
with the current set of options.  This commit adds 2 new options and 2
new highlight groups.

Specifically, we can now

1.  differentiate between changes, deletes AND signs that have both
    with the `signify_sign_changedelete` option.
2.  disable showing the number of deleted lines in the sign column with
    the `signify_sign_delete_use_count` option.
3.  highlight lines with changes and deletes specifically with the
    `SignifySignChangeDelete` highlight group.
4.  highlight the first line differently with the
    `SignifySignDeleteFirstLine` highlight group.

All of these default to existing settings/highlight groups, so
everything should continue working the way it does now.

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For those that are curious, I wanted to configure signify to show no
symbols in the sign column, and to just use the highlight groups to give
me all the information I needed.  I am able to achieve that now with the
following settings and the Solarized colorscheme:

let g:signify_sign_add = "\<Char-0xa0>\<Char-0xa0>"
let g:signify_sign_change = "\<Char-0xa0>\<Char-0xa0>"
let g:signify_sign_changedelete = "__"
let g:signify_sign_delete = "__"
let g:signify_sign_delete_first_line = "‾‾"
let g:signify_sign_delete_use_count = 0

hi SignifySignDelete term=bold,underline cterm=bold,underline ctermfg=1 ctermbg=12
hi SignifySignDeleteFirstLine ctermfg=1 ctermbg=12
hi SignifySignChangeDelete term=bold,underline cterm=bold,underline ctermfg=1 ctermbg=3

You can see it in action here:

https://twitter.com/_bentomas/status/486266117204090880
Introduce g:signify_sign_show_count

This option replaces g:signify_sign_delete_use_count and works for both,
delete and change/delete.

Referenes #125.
Doc: add g:signify_sign_show_count

References #125.