Add config for client

[david4096]

ga4gh/ga4gh-server#537

Typing urls, etc. on the command line to launch the client is tedious. Ideally there should be some kind of optional client configuration file that can be read in and take e.g. the following options:

• server url
• minimalOutput flag
• verbosity
• logging options (log to file? etc.)

This is similar to how you might use the s3 client, where it gathers your key info from environment or configuration files.