Add '--type' command line option.

The first step in supporting exporting from multiple different services
is to create the command line syntax to specify which type (jira,
github, etc.) the server endpoint should be treated as.

To support this, this commit adds the --type argument.  Only 'github'
or 'jira' are acceptable values here.  Currently, jira exporting is
supported.  Specifying '--type github' currently only displays a
message saying that github exporting isn't supported yet.

Issue #7