Make -c optional

[maximilien]

Currently for each i18n4go command the user needs to use -c before the command name. With COBRA support we can remove this, so for instance:

$i18n4go -c extract-strings -f i18n4go/i18n4go.go -v

should now be

$i18n4go extract-strings -f i18n4go/i18n4go.go -v

Also, since we will be deprecating -c let's add a warning when user still have -c like:

WARNING: the -c before <command-name> is now DEPRECATED and no longer needed. Future versions of i18n4go will remove -c alltogether

or something similar.

[maximilien]

Fixed with #89