Autodetect gregorio executables
Autodetect gregorio executables

Gregorio executable might have several names :

- gregorio for default executable in TeXLive ;
- gregorio-?_?_? for other versions available in the system.

This commit lets gabc.py autodetect those executables, instead of
hardcoding a list each time Gregorio is updated.

AugustH/pandocfilters,jgm/pandocfilters