fix broken --langmap= parameter parser

This patch fixes a bug reported at
https://github.com/fishman/ctags/issues/71
by Daniel Hahler.

Offset calculation is inconsistent inside and outside of
extractMapFromParameter() when multiple extensions are
given to --langmap= option.

More Unit tests for --langmap are added, too.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>