Support "mvnvm --home" (or similar) to output the MAVEN_HOME directory

[Nicolas Venegas]

In some cases you may need to pass the MAVEN_HOME value as an environment variable or command argument, so being able to use <backtick>mvnvm --home<backtick> in its place would be awesome.

[mjensenbitbucket-bot]

This has been done here: https://bitbucket.org/mjensen/mvnvm/commits/1dd4ad691e653ee64aecb1de933b98063b162638