spack python: add -c option

Allows passing program in as a string. Example:

    $ spack python -c 'print 2+3'
    5

Also imports spack module by default into the environment.

lib/spack/spack/cmd/python.py

@@ -31,6 +31,8 @@
 import spack
 
 def setup_parser(subparser):
+    subparser.add_argument(
+        '-c', dest='python_command', help='Command to execute.')
     subparser.add_argument(
         'python_args', nargs=argparse.REMAINDER, help="File to run plus arguments.")
 
@@ -38,7 +40,8 @@ def setup_parser(subparser):
 
 def python(parser, args):
     # Fake a main python shell by setting __name__ to __main__.
-    console = code.InteractiveConsole({'__name__' : '__main__'})
+    console = code.InteractiveConsole({'__name__' : '__main__',
+                                       'spack'    : spack})
 
     if "PYTHONSTARTUP" in os.environ:
         startup_file = os.environ["PYTHONSTARTUP"]
@@ -47,7 +50,10 @@ def python(parser, args):
                 console.runsource(startup.read(), startup_file, 'exec')
 
     python_args = args.python_args
-    if python_args:
+    python_command = args.python_command
+    if python_command:
+        console.runsource(python_command)
+    elif python_args:
         sys.argv = python_args
         with open(python_args[0]) as file:
             console.runsource(file.read(), python_args[0], 'exec')