Allow access to additional arguments passed to synthtool (#166)

These can be accessed via:

```
from __main__ import extra_args

print(extra_args())
```

or

``1
from synthtool.__main__ import extra_args

print(extra_args())
```

synthtool/__main__.py

@@ -15,6 +15,7 @@
 import importlib
 import os
 import sys
+from typing import List, Sequence
 
 import click
 import pkg_resources
@@ -29,11 +30,39 @@
     VERSION = "0.0.0+dev"
 
 
+_extra_args: List[str] = []
+
+
+def extra_args() -> List[str]:
+    """Return any additional arguments specified to synthtool."""
+    # Return a copy so these don't get modified.
+    # A little trickery. If __name__ isn't __main__, import __main__ and return
+    # its extra_args(). This ensures that both `from __main__ import extra_args()`
+    # and `from synthtool.__main__ import extra_args()` works as expected. This
+    # is needed because *technically* Python can have two copies of this module
+    # in sys.modules when it's executed as main. Weird, I know.
+    if __name__ != "__main__":  # pragma: no cover
+        try:
+            import __main__
+
+            return __main__.extra_args()
+        except AttributeError:
+            # __main__ didn't have an extra_args() attribute, so this means
+            # synthtool is not the main module. Just return what's in this
+            # module.
+            pass
+
+    return list(_extra_args)
+
+
 @click.command()
 @click.version_option(message="%(version)s", version=VERSION)
 @click.argument("synthfile", default="synth.py")
 @click.option("--metadata", default="synth.metadata")
-def main(synthfile, metadata):
+@click.argument("extra_args", nargs=-1)
+def main(synthfile: str, metadata: str, extra_args: Sequence[str]):
+    _extra_args.extend(extra_args)
+
     synthtool.metadata.register_exit_hook(outfile=metadata)
 
     synth_file = os.path.abspath(synthfile)
@@ -43,7 +72,12 @@ def main(synthfile, metadata):
         # https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly
         spec = importlib.util.spec_from_file_location("synth", synth_file)
         synth_module = importlib.util.module_from_spec(spec)
+
+        if spec.loader is None:
+            raise ImportError("Could not import synth.py")
+
         spec.loader.exec_module(synth_module)
+
     else:
         synthtool.log.exception(f"{synth_file} not found.")
         sys.exit(1)