Convert ZSH_HIGHLIGHT_STYLES to zstyle

[danielshahaf]

Idea: instead of assigning to a global parameter (ZSH_HIGHLIGHT_STYLES[myhighlighter:aurora]=fg=green), set a zstyle; e.g.:

# :<fixed string>:<format number>:<colors>:<highlighter name>:<colorstyle (key)>
zstyle :z-sy-h:0.5:${terminfo[colors]:-unknown}:myhighlighter:aurora color fg=green

Benefits:

• Can easily set the same colorstyle (aurora in this example) for multiple highlighters
  
  [an aurora colorstyle might exist in multiple highlighters; that's why colorstyles are namespaced (0ff354b)]
• Can use zstyle -e to set colors dynamically (based on $HOSTNAME, date and time, or even retrieved from a cloud service)
• Can easily write a theme that works in multiple settings (light/dark terminal background, 88/256/more colors, etc)

The styles would be set by themes or by the user in zshrc, and read in the highlighters as they populate $region_highlight.

As to the themes branch (#241) : perhaps the theme should be part of the context? Then, highlighters could look up color values in the context :z-sy-h:…:${ZSH_HIGHLIGHT_THEME}:…, and users could change the active a theme simply by assigning to $ZSH_HIGHLIGHT_THEME.

(Terminology note: I'm going to call aurora a "colorstyle" and the color a "zstyle"