Robustness improvements

[BryanQuigley]

Require yaml parsing for build, builds will fail if yaml
isn't correct.

switch to faster megalinter which should make it easier to do
more future tests

included some yaml prettier changes

Resolves: #276

[BryanQuigley]

I had a better version of this megalinter check on I guess a private project, but this still should be much faster to use locally.

[timwis]

Hey @BryanQuigley pardon my delay - I've just got back from holiday. This looks fine to me - the only thing we'd want to check is whether the way that DecapCMS saves YAML is compatible with these linting rules, otherwise the build would fail after every update from the CMS. Unless this only checks source code and not the data directories?

[BryanQuigley]

Thanks for the review! Hope you had a good holiday.

Confirmed the DecapCMS saved yaml works fine. The megalinter yaml check is likely easier to pass then the Jekyll requirements (or it's just formatting)- which if those don't work we don't get a page.

DecapCMS seems to work a bit better locally now - going to see if I can automate that more..