[tools] Adopt a2cho for CordPro 6

[tools] Adopt a2cho for CordPro 6

Alter order of replacements for environments: introduce '-- Start of
...' instead of '{start_of...'. ChordPro recognize '^--' as comment,
and that is left untouched.

Conversion is a multi step process: a-txt --> inter-txt --> inter-cho
--> cho. Chordpro converts between the intermediary representations
from ascii text to chordpro format.