Add p.append and p.prepend translator combinators

[guillaumearm]

I think we could add append and prepend combinators for enhanced wording.

• p.t is a translator
• p.append is a combinator
• p.append('...').t is a combined translator

examples :

p.append(' ...').tc('home.common.title');
p.prepend('-> ').tc('home.common.title');

p.prepend('-> ').append(' ...').tc('home.common.title')

In the future, we can imagine export more combinators in a new addons/redux-polyglot-combinators.

I think we should open another issue for talking about replace p.tm() by a combinator. please see #62