Add --resolve-only option

When the option is provided, only the workspace is created and the location is printed to the command line. This can then be used, for example, as classpath for another tool.

```shell
$ jgo --resolve-only org.scijava:parsington
/home/hanslovskyp/.jgo/org/scijava/parsington/RELEASE
$ ls /home/hanslovskyp/.jgo/org/scijava/parsington/RELEASE
mainClass  parsington-1.0.4.jar  pom.xml
```

This is experimental (for now) but might be merged at some point in the future

Author: hanslovsky

Diff for: jgo/jgo.py

@@ -271,6 +271,7 @@ def jgo_parser():
     parser.add_argument( '--additional-endpoints', nargs='+', help='Add additional endpoints', default=[], required=False)
     parser.add_argument('--ignore-jgorc', action='store_true', help='Ignore ~/.jgorc')
     parser.add_argument('--link-type', default=None, type=str, help='How to link from local maven repository into jgo cache. Defaults to the `links\' setting in ~/.jgorc or \'auto\' if not specified.', choices=('hard', 'soft', 'copy', 'auto'))
+    parser.add_argument('--resolve-only', action='store_true', help='Only resolve dependencies and print workspace location. Will not run any code.', required=False)
 
     return parser
 
@@ -552,6 +553,12 @@ def run(parser, argv=sys.argv[1:], stdout=None, stderr=None):
         shortcuts           = shortcuts,
         verbose             = args.verbose,
         link_type           = link_type)
+    if args.resolve_only:
+        print(workspace)
+        class MockObject(object):
+            def check_returncode(self):
+                pass
+        return MockObject()
 
     main_class_file = os.path.join(workspace, primary_endpoint.main_class, 'mainClass') if primary_endpoint.main_class else os.path.join(workspace, 'mainClass')