Add text domain option

[rmccue]

For plugin authors, it's helpful to only have Pig Latin activated for certain text domains, rather than all. This helps to make sure that not only is the text translatable, but it's also in the correct domain.

I'd like to add a new global variable that can be used for this, $piglatin_domains = array('default'), which can be overridden by plugins or in wp-config.

[nb]

Limiting domains sounds good.

Why not a filter, instead of a global variable? No translations should be happening before init anyway.

[rmccue]

Why not a filter, instead of a global variable? No translations should be happening before init anyway.

With a global variable, I can add it as a one-liner to my wp-config when developing, rather than needing to write a mini-plugin to do it.

[nb]

OK, makes sense.