Ability to specify config file

[hrj]

It should be possible to specify a different path for the config file from the command line.

Hence, Config should be converted to a class, which takes file path as a constructor parameters.

[hrj]

1. The user can specify the config path on the command line. For now, the first argument on the command line can be the config file path.
2. The default path for a config file, when not specified on command line, is data/config.json
3. If a file doesn't exist at the config path, a default one is created at that path, and read back as the current config

2. and 3. are already done, only 1. needs to be implemented

@rr83019 This plan is a little different from what we discussed. The change to in-memory database can be done separately later. Trying to keep the changes to a minimal for now.