Move Webpack parsing to Utils, enable for ERB templates

[jaredcwhite]

Hey @ParamagicDev, I relocated the bulk of the code that parses the manifest to the Utils module so we can use it from ERB/etc. as well. I also changed the behavior so it doesn't actually trigger any runtime errors and always lets the site build…I think there must have been a communication mixup before and I thought that's how it was already working. Anyway, if you could give this a quick looksee before merge, I'd appreciate it!

(Also fixed an issue with using Liquid filters from ERB…had to add duck-typing for a @context ivar so the filters could access the current site object).

[KonnorRogers]

Looks good to me, no more unknown Liquid Exception (hopefully). I do like the idea of the parsing happening in a util. Also updated tests is always nice :)