mBuild 0.9.0

Features

• A plugin or "recipe" architecture has been added to allow external modules to be imported inside of mBuild (#501)
• Python 3.6 is now explicitly supported and tested (#518)

Maintenance

• A contributor's guide (#500) and .github issue & pull request templates (#498) have been added
• A redundant and unused block of code was removed (#515)
• The glotzer and bioconda channels, which are now obsolete in this scope, have been dropped (#516)