hrc/sh.hrc

I'd like to suggest 3 little improvements for colorizing shell scripts in the editor:

1. variable expansions: ${#...} ${!...}
The first one is colorized as a comment. I think the better way to consider both ${! and ${# as the start of block the same way as ${.

2. processing of the modern bash brackets such as (( )) and [[ ]]
It could be more convenient if (( )) and [[ ]] were considered as paired brackets (used in modern BASH) instead of two separate nested brackets ( ) and [ ]. 

3. the better support of heredoc
The marker is not restricted by EOxxx as in the previous version; also added support of <<-marker.