Add support to download maven from a central mirror

[Patrick Roumanoff]

Most organisation maintain a mirror of central and central is hosting maven disitributions. In a CI environment ability to use a central mirror will improve build time and reliability.

I am proposing to add a --central-mirror option to provide the url of said mirror (typically a nexus instance)

[mjensenbitbucket-bot]

https://bitbucket.org/mjensen/mvnvm/commits/96ef781e7018e6c7a04df949f61da1ca96c58aa7