Add feature: sbt upgradeBuildFile

[shigemk2]

I add a feature to upgrade build file.

sbt-update does not have a feature to upgrade build file just like npm-check-updates:
https://github.com/tjunnone/npm-check-updates

npm-check-updates has a feature to upgrade package.json when some packages have update.
So I wanted a feature to upgrade build file.

We can upgrade build file by sbt upgradeBuildFile.

Default build file is build.sbt, but there are many projects whose build file is not build.sbt. So I let them to set build file location in dependencyUpdatesReportFile.

[rtimush]

Thank you for the pull request! I will take a look as soon as I have some time.

[shigemk2]

Thank you for your comment. Please review my PR!

[rtimush]

Thank you again for your contribution. While I think that upgradeBuildFile may be useful, there are some concerns:

1. Dependency versions are often declared as variables, upgradeBuildFile will not work in such cases. This is understandable but limits the command usability.
2. Build file update process based on string match and replace is very fragile. Sometimes it will fail to update a dependency (say, if there is a line break), sometimes it will result in incorrect file updates, which is much worse.

Given that, I think that upgradeBuildFile in this implementation will result in constant uncertainty whether it will work this time or not, so I tend to refrain from merging this pull request.

[jozic]

@shigemk2
probably bunch of (passing) scripted tests can address concern 2

[shigemk2]

I am very sorry. I forgot this PR's comment.
I am not sure whether I resolve two concerns you showed now.
So, this PR is incomplete.