Fix install.py --mcp-dir option

The --mcp-dir command-line option to FML install.py always fails with:

 $ py install.py --mcp-dir
Traceback (most recent call last):
  File "install.py", line 33, in <module>
    mcp_dir = os.path.abspath(options.mcp_dir)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 343, in abspath
    if not isabs(path):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 53, in isabs
    return s.startswith('/')
AttributeError: 'bool' object has no attribute 'startswith'

This PR changes the argument to 'store' instead of 'store_true', allowing the custom MCP dir to be passed without error.