add separate path_prefix and path_approx hilights
Issue #92 Eliminates system bell errors on osx.

Prepended an 'if' to remove error bell on osx systems.
dollar variable and backslash codes highlighting
Update main-highlighter.zsh

Add support of CDPATH
Update main-highlighter.zsh

Comments added
#155: document path_prefix/path_approx
Customize highlighters.