Add base_branch to exported envs

[iamogbz]

Currently it does not look like there's a way to get the base branch being targeted in the case of a pull request.

[siddharthkp]

Which CI platform are you on?

Does this bit help?: https://github.com/siddharthkp/ci-env/blob/master/index.js#L19-L22

[iamogbz]

@siddharthkp want to use it for a tool that could be run on any env. Looks like the code will only tell you current branch.

ci-env/index.js

Lines 19 to 22 in cc891b5

	branch =
	process.env.TRAVIS_EVENT_TYPE === 'push'
	? process.env.TRAVIS_BRANCH
	: process.env.TRAVIS_PULL_REQUEST_BRANCH

When running from a pull request e.g.

It would tell you develop, but I want to also know which branch is being targeted i.e. master

[siddharthkp]

ah, I understand now. Does travis/other CI expose that information?

[iamogbz]

@siddharthkp travis has TRAVIS_BRANCH https://docs.travis-ci.com/user/environment-variables/, I think github has GITHUB_BASE_REF. Not certain about others, but hopefully they would

[siddharthkp]

Perfect, would you like to shot on the implementation? It might make sense to keep the name similar to pull_request_number