Enable config file parsing

We support 4 different config files:

~/.mitmproxy/common.conf:       Options that are common to all tools
~/.mitmproxy/mitmproxy.conf:    Options for mitmproxy
~/.mitmproxy/mitmdump.conf:     Options for mitmdump
~/.mitmproxy/mitmweb.conf:      Options for mitmweb

Options in the tool-specific config files over-ride options in common.conf. If
a non-common option is put in common.conf, an error will be raised if a
non-supporting tool is used.