#38 ``keyword'' and ``h_state'' coloring priority

[Max]

Currently ``h_state'' and ``keyword'' matching the same
pattern, ``h_state'' is used. Sometimes such behavior
may be unwanted.

There should be a way to specify keywords that have
priority over ``h_state'' statements.

I propose to introduce a directive like
``const_keyword'' with a coloring in order:
1) all keywords from ``keyword'' definitions
2) all ``h_state'' coloring
3) all keywords from ``const_keyword'' definitions

In other words, ``const_keyword'' would define keywords
that can not be re-colored even if matched by some
``h_state''.