Add ability to format java code with a custom google-java-format version

[chingor13]

Usage would be something like:

import synthtool.languages.java as java

# ... do some codegen into ./src
java.format_code("./src")

Note the gapic-generator does run version 1.1 of the Google java formatter and this one defaults to 1.6 (most up to date). We can update the one in gapic, but we'd also like the ability to run this on our apiary libraries too.