0.3.0 - Artifact pinning and override targets

Grazel now supports rules_jvm_external's artifact pinning and ability to generate override targets in WORKSPACE's maven_install rule.

Docs:

• https://grab.github.io/Grazel/grazel_extension/#override-targets
• https://grab.github.io/Grazel/grazel_extension/#artifact-pinning

Changes

• Maven artifact pinning and override targets by @arunkumar9t2 in #10

Full Changelog: 0.2.0...0.3.0