Download config file permanently instead of using a temp file.

This might be preferable, because you won't have to manually download the
scalastyle-config.xml file if you want to work without a network connection.
On the other hand you will want to add it to your .gitignore.
Redownload if config is older than a day so you stay up-to-date mindlessly
Fix off-by-one error and erroring if can't download config even if it's already local
Make scalastyleConfigUrl an Option and have default behavior not to download.

Instruct user in error message when config is not local and url is not
set that one or the other must be done.

(cherry picked from commit 64b82f31592466ad798e134dae41e558fa0b51c0)

Conflicts:
	src/main/scala/Plugin.scala
Configurable time until fetch from remote happens again.

Key is scalastyleConfigUrlRefreshTime. It's in hours, and a negative integer
will indicate to never fetch (good for when you are testing local changes.)
Update README to mention the remote configuration option

@owners: I will leave it up to you to update the version number in the README.

(cherry picked from commit 5efdfe83b12bd490b2708b6f1e5a3fe04d5179b8)
Merge branch 'download-config'

Conflicts:
	README.md
	src/main/scala/Plugin.scala