WIP: Highlight history modifiers (#485)

[danielshahaf]

This is #485 (comment), rebased and with the tests written (for reasons explained there).

Concerns:

• Highlight parameter expansion history modifiers #485 (comment) "Modifiers require braces when KSH_ARRAYS is set."
• Highlight parameter expansion history modifiers #485 (comment) Modifiers with arguments
• The :t2/:h2 modifier (new since Highlight parameter expansion history modifiers #485) - https://github.com/zsh-users/zsh/blob/e5765bfdc89f131d03051ba4963819fdfe252503/README#L49-L63
• Highlight parameter expansion history modifiers #485 (comment) @phy1729 requested that "[some unknown referent] and the parameter handling in _highlight_arguments should be pulled out into a new function" — @phy1729, do you by chance recall what the words "that bit" referred to?

[danielshahaf]

Triage: incomplete, new feature (as opposed to bugfix), 0.8.0 has plenty of content already merged, redrawhook is the next priority: demilestone.