RenameCommand 0.4.0

Added file extension parameter to the rename function, added try option, minor fixes.

Added the file extension parameter in case the function needs to rename differently depending on the extension. The try option is a little like dry-run but doesn't require the file to exist. Removed Linux platform for now because it seemed to stop working as of Xcode 12. Minor fixes to file path and extension parsing that were probably wrong before. Updated the readme to include a clearer description of using swift-sh and of the Regex additions.