Function undercurl was merged with function hi, which gained additional
argument for passing special gui attribute (underline, undercurl, bold,
strikethrough etc). Perhaps it might be a good idea to pass the
attribute to cterm either.
There was some check regarding specific features Vim have, like
spell check which was introduced with Vim 7.0, and even on old
repositories, the oldest version of Vim I could find is 7.4. Let's get
rid of unnecessary legacy.
Nowadays, all significant terminals are capable for displaying at least
256 colors, that's why I decided to drop support for old 88 color, since
the approximation is pretty inaccurate for such terms.
