Add Flag to Set User Agent

[kevinjfoley]

Charles Schwab requires headers to be set to None otherwise the request times out.

Quick fix is to set DEFAULT_USER_AGENT in client.py to None, then add the Schwab account and switch the value back.

I plan on opening a PR to add logic similar to

ofxclient/ofxclient/cli.py

Lines 242 to 244 in 6d6db69

	if 'ofx.discovercard.com' in bank_info['url']:
	# Discover needs no User-Agent and no Accept headers
	client_args['user_agent'] = False

to handle it but wanted to open this in case others run into the issue in the meantime.