Initial version of Golang with refined interface

July 2019 Update

The oauth2l client has been redesigned to support a richer set of
command-line options. Options such as "scope", "audience", and "email" can
now be specified explicitly in order to avoid ambiguity and mis-use.
Furthermore, several options have been deprecated and/or renamed for clarity:

• --json is now --credentials
• --credentials_format is now --output_format
• --jwt is now --type jwt
• --sso is now --type sso

Note: The tool maintains backward compatibility with the old option names
and continues to accept implicit options for "scope", "audience", and
"email" based on context. However, please use the new style of explicit
options going forth. Refer to the "Command Options" section for details.

Please check the README file for more information about the usage.