Add a loadComponents function for easy component loading on NodeJS

[Golmote]

This PR adds the loadComponents function mentionned in #972.

Usage:

var Prism = require('./components/prism-core.js');
var loadComponents = require('./components/index.js');
loadComponents(['c', 'javascript']);

[Golmote]

@mAAdhaTTah Do you wish to do a quick review on this?

[mAAdhaTTah]

Maybe loadLanguages would be more appropriate as a name, since it can't load other components, like themes or plugins?

Also, we get slightly less code if we just call load within the if block, e.g.:

if (components.languages[language] && components.languages[language].require) {
	loadComponents(components.languages[language].require);
}

[Golmote]

Yes ! Both valid points. I updated the PR.

[mAAdhaTTah]

LGTM!