merge current build context environment vars for lambda build

[shmargum]

resolves #59

[8eecf0d2]

Might be worth splitting out to a context merging method to set all options (not just environment) with support for branch specific contexts and the special context branch-deploy. Don't see a need to support deploy-preview since PR's aren't a thing locally and would use the special branch-deploy context if pulled anyway.

Based on docs, given the following config and run within a git branch called foo-branch:

[build]
  publish = "/default"
[build.environment]
  SOME_VAR = true
[context.branch-deploy]
  publish = "/branch-deploy"
[context.branch-deploy.environment]
  SOME_VAR = false
[context.foo-branch]
  publish = "/foo-branch"
[context.foo-branch.environment]
  SOME_OTHER_VAR = 10

We should end up with:

{
  "build": {
    "publish": "/foo-branch",
    "environment": {
      "SOME_VAR": false,
      "SOME_OTHER_VAR": 10,
    }
  }
}

[swyxio]

sorry for late response - i was on holiday - i see theres also some desire to use the BRANCH env var: #59 (comment) - but i dont see that as a blocker to merge this.

up to your discretion on this one @8eecf0d2, i dont feel too strongly about it

[shmargum]

i've updated this to use the branch config
i'd love to add a test but theres no testing framework set up yet

[swyxio]

awesome - ive been wondering about tests too and should probably get that going at some point. just wasnt built into it from the start.

[shmargum]

I've added some tests as well as a travis yml in #94
don't know if this is what you had in mind; but you can see the travis build successfully running the test on my fork:
shmargum#1
if an admin for this repo enables travis ci then you would also see the tests run on #94

[swyxio]

wow! nice! very kind of you! unfortunately i'm not an owner of the netlify org so i have to request permission to add travis - just requested it but no guarantees

[shmargum]

This is included in #94