colors/szakdark.vim: a couple minor color tweaks and attribute changes

The String color was a bit bright, while rubyStringDelimiter,
CursorLine, and CursorColumn were a bit too dark.  Also made Title bold.
vimrc: add a helper for appending to the buffer local snippet dirs
Merge branch 'add-snippet-helper' of https://github.com/jszakmeister/vimfiles