[Mediawiki-api] adding action=getmsg

[Michael Dale]

Could we add in an action get message to the api or add some 
functionality to meta=allmessages ?

Really need to be able to:
A) send in parameterized replace values per msg key
B) get the html rendered / parsed result (rather than the wikitext)   

Security wise the parameterized replace values should be treated as 
wikitext and go through the wiki-text-parser before rendered back out to 
the browser.

Otherwise we have to do quite a few calls to generate a msg text that 
has a parametrized wiki-text-value.

--michael