Add command to upgrade configuration file version

[kyleconroy]

As we plan on shipping our second configuration version in #302, we're going to want to offer users an easy way to convert their configuration to the latest version. The Go tool offers similar functionality via go fix, so I think we should use that name.

sqlc fix will look for sqlc.json in the current directory. If it exists, it will check the version of the configuration. If the version is not the most recent, it will parse the existing file, convert it to the most recent version, and write it back to disk. If the --diff flag is specified, the new contents of the configuration file will be printed to stdout instead of being written to disk.

[kyleconroy]

Closing this as we have continued to support both versions of the configuration file. Will revisit this if we ship a V3 of the configuration file.