Support transpilation with Babel

[littledan]

Allows Babel to be used as a transpiler, before passing to the underlying host, with the command-line flag --babelPreset stage-2, for example.

Depends on this patch bterlson/eshost#19 , so eshost would have to rev a semver-minor and this patch would have to depend on that, before landing.

[rwaldron]

@littledan can you rebase this? thanks!

[littledan]

Rebased, sorry for the delay here. @bterlson Could you take a look?

[bterlson]

LGTM!

[rwaldron]

@littledan This depends on bterlson/eshost#19, right?

I will land this once travis goes green.

[bterlson]

Yes it does.

[leobalter]

@littledan status? I would love to use this feature.

[leobalter]

I guess the only missing part is bumping eshost to ^3.5.0? Let me check this

[leobalter]

Running the tests locally, before and after rebasing (with clean npm installs) with the master branch. Makes the CI failures pass:

# Test requiring transpilation
ok 93 Test has an "expected" frontmatter
ok 94 Test passes or fails as expected
ok 95 should be equal
# Test requiring transpilation (Strict Mode)
ok 96 Test has an "expected" frontmatter
ok 97 Test passes or fails as expected
ok 98 should be equal

[littledan]

Bumped the dependency; we'll see what CI says

[bterlson]

I think the only issue here is that those tests need to be async (see #85). Let's fix #85 and confirm.

[rwaldron]

Merged!

[littledan]

Thanks!