Support passing -c/--config arg

[rkok]

Allow for passing a custom config file using the -c or --config argument.

In my case, this came in useful for scripting Transferwise (Wise) imports, where the files don't always have the same currency:

ofxstatement convert \
  -t transferwise \
  -c <(echo -e '[transferwise]\nplugin=transferwise\ncurrency=EUR') \
  statement.csv out.ofx

[coveralls]

coverage: 95.492% (+0.005%) from 95.487% when pulling 4791986 on rkok:feature/config-arg into 9f38a90 on kedder:master.

[kedder]

Makes sense, thanks!