Add REBOL detection and (basic) parsing

Add REBOL detection and (basic) parsing

Also adds a simple .r disambiguation to discern REBOL and R sources. R
is the default, REBOL is used if "rebol" is found anywhere in the
contents.

The REBOL parser currently does not handle multi-line strings ({...}),
which could (in rare cases) lead to string parts being classified as
comments.

Signed-off-by: Andreas Bolka <a@bolka.at>