Improve usage of custom Prism themes

[gregone]

As I found when working on https://github.com/MelSumner/ember-a11y-for/pull/14 , using a non-standard theme (i.e not in the main prismjs package) is quite convoluted.

Having the option to not include any css when explicitly using 'none' instead of the default theme would help some of these issues.

see: https://github.com/shipshapecode/ember-prism/blob/f352008fb699ea05a1554140902c71798419e007/index.js#L23

[RobbieTheWagner]

@gregone would you be interested in opening a PR to fix the issue?

[gregone]

@rwwagner90 Sure, I can do that. Would it be ok if made theme: 'none' work as expected, i.e no css at all?

[RobbieTheWagner]

@gregone could we do theme: false instead perhaps?