Adds an --extension option to generateJsonI18n

Usage:
	php maintenance/generateJsonI18n.php --extension WikiLove
Performs the default conversions in $IP/extensions/WikiLove/, replacing
the WikiLove.i18n.php file with a backwards-compatibility shim and porting
messages to the WikiLove/i18n/ directory.

This patch has the side-effect of making the second cmdline argument in
the ordinary usage optional.  When omitted, we will make the default guess
as to $jsondir, that it should live in the same directory as the .i18n.php file.

Change-Id: I6246d2c4eb7327d6dcc503421b71ffdcaf01769c