Make home and binary location more discoverable

[bkhouri]

After reading https://swiftpackageindex.com/swiftlang/swiftly/0.4.0/documentation/swiftlydocs/automated-install, one improvement to the init could be to move the SWIFTLY_HOME_DIR and SWIFTLY_BIN_DIR from environment variables to command line options to make them more discoverable.

These values of both can optionally be included in the configuration, if it's not already