Avoid hardcoding port 8093 to allow multiple instances of Glyphhanger to run in parallel

[jialiang]

Currently the Web Server is hard-coded to always use port 8093: https://github.com/zachleat/glyphhanger/blob/ef6485b1d84b55234dc13e493cb85676e37dae0b/src/WebServer.js#L8

Would be great if it can either:

• Check if the port is already in use and try the next port number instead or
• Allow passing a custom port number to use, e.g. --port 1234

My use case is to have multiple instances of Glyphhanger running in parallel to subset multiple font files.