Add offset support to highlighter

[Thesola10]

Currently, the highlighter included in this plugin does not support non-zero starting positions in commands, which means two things:

1) If a command is padded with whitespace, which is valid zsh syntax, the highlighter will color it as if from the start. 2) The highlighter will not work on chained commands (e.g. after ; or ||).

Need to study how zsh-syntax-highlighting does it.

[Thesola10]

This is looking more and more like a job for an external helper -- having data to identify where all commands are would allow me to do more (like provider package on hover) without an explicit dependency on zsh-syntax-highlighting