Add config file

[mikafinja]

Addes a configuration file as described in issue #8 (and a .gitignore file)

[pklaus]

Great idea, to put the current DEFAULT_... settings into a config file.

Here are my thoughts:

• I think, loading the config file should not be either custom or default. I would prefer to always load the default config plus the custom one (if available) where the custom one would only overwrite the settings specified in there. As an example, the custom config file could only consist of a single line specifying the printer string in case the other defaults are acceptable.
• While JSON is a nice format for machines, I'm not a huge fan when it comes to config files. I think INI or YAML are more readable and writable for humans.
• The custom config file could resided outside of the repository. A good place would be ~/.brother_ql_web. It should be possible to change the location where brother_ql_web is looking for the config file with a -c command line parameter (and optionally with an environment variable).

I'm not fully decided yet. I think I will merge the changes and then rework it according to the ideas mentioned above.