Add support for excluding YAML frontmatter

Add support for excluding frontmatter settings common in many static
site generators like Middleman and Jekyll. It's controlled by a new
global config option 'skip_frontmatter', which defaults to false so it's
opt-in.

Change-Id: I664e618c8acc3c49aa65686579b5fec7e2e1aeba
Reviewed-on: http://gerrit.causes.com/42551
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Indent heredocs in Parser spec

Heredocs that aren't indented are more difficult to read. Fix that by
indenting them and using the `normalize_indent` helper to avoid a parse
error.

Change-Id: Ia8cde89109290a548f421ad1db642e9c7926c7a4
Reviewed-on: http://gerrit.causes.com/42552
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Relax Parser raise_error expectation

Checking for a specific error string is brittle unless we are
intentionally verifying the contents of the string because it is
something we generate ourselves.

Relax the check by merely verifying that a `Haml::Error` was raised.

Change-Id: I28cab926e5818d6532bea6be87eb5fbb16f06042
Reviewed-on: http://gerrit.causes.com/42553
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>