Make plugin configurable via dotted `.sourcegraph-jetbrains.properties`

Created by: XanderXAJ

Normally configuration files for tools (especially global configuration) tend to begin with a dot to avoid showing up in file listings and other tools (unless desired).

This change adds .sourcegraph-jetbrains.properties as a candidate path to check for the global configuration file. Since dotted files are conventional for configuration, the path mentioned in the README has been changed to suggest the dotted file name.

The original non-dotted filename is still read as a backup for backwards compatibility.