Make filter accept a single object

[gabegorelick]

Work in progress

• contexts
• plurals
• make sure extraction is possible
• more tests

Example use:

<div attr="{{ 'Message' | translate:'{context: \"ctx\", plural: \"\\{\\{$count\\}\\} boten\", n: \"count\"}' }}"></div>

Continuation of #122

[gabegorelick]

As I mentioned [1], this PR changes the existing API because Angular doesn't support passing object literals to filters in expressions. But there have been no releases with the existing translate filter API, so it shouldn't break anything.

[1] rubenv/angular-gettext-tools#63 (comment)

[gabegorelick]

Yeah, this is pretty gross looking. Might have to resort to some hacks to work around Angular's suckiness.

[gabegorelick]

#150 looks more promising