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.

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