Adding support for artifacts

refs: https://github.com/travis-ci/travis-artifacts/issues/28
      https://github.com/meatballhat/artifacts
Ensuring the artifacts binary is in $PATH
Changes per feedback on #242
Validating required artifacts params, overriding controlled params
Silencing stderr from initial artifacts check
Removing initial check for `artifacts` binary
Rearranging some of the artifacts addon's steps and echoed messages
Allowing echo of *some* artifacts env vars
More artifacts UX cleanup per feedback
A few more artifacts addon tweaks based on @joshk conversation :smiley_cat:
Breaking out non-runnable messaging, constantizing some settings for artifacts addon
Raising artifacts max-size to 50MB, handling branch config better

per feedback on #242
Adjusting messaging around current branch being runnable in artifacts
Double-quoting all artifacts env vars

so that the array-ish values that are rolled into ";"-delimited strings
don't result in unexpected executions, mostly...  and also for "safety" (?).
Using colon instead of semicolon for multi-value artifacts env vars

to match up with its current behavior
Merge pull request #242 from meatballhat/artifacts-support

Adding support for artifacts