Allow _meta patterns to support any engine

[AndrewLeedham]

Addresses: _meta patterns being forced to use mustache.

Summary of changes:

• Replaced ".mustache" literal with patterlab.config.patternExtension.
• Allows the use of other engines like handlebars for _meta files.

[bramsmulders]

Great idea, but this wouldn't work in some engines. For instance the react engine cannot spit out an incomplete dom tree.

[dave-cross]

Curious if this _meta section would be a better fit in pattern engines themselves. This should allow engines to have better control over their respective file formats. A single method in the engine that either calls multiple meta files or one (react) wrapper file.

Possible PL3.0 wish?

[bmuenzenmeyer]

@cloudsociety
you are right on target. 🎯

this is basically the idea behind #611 which is planned for 3.0

I refrained from commenting on this PR until I could think through how (or if at all) patterlab.config.patternExtension factors into that solution

[AndrewLeedham]

This was more of a hot fix for a project I'm working on, that I thought I'd create a PR for, so I didn't consider all outcomes. I also noticed that the styleguidekit extensions are fixed as .mustache, so I can't use styleguidekit-handlebars-default.

[dave-cross]

@bmuenzenmeyer Awesome! Looking forward to it!

@AndrewLeedham I'll still steal this bit of code so my patterns can build under a single engine in 2.x (having too much fun with Twig/Nunjucks to return to Mustache). Thanks for the share.

-Dave

[bmuenzenmeyer]

since this is in the spirit of what we want to do, but the wrong implementation, i am going to close this for now. PL Node 3 will handle this with my above approach