[Feature] Add a ConfigManager class

[sarthakpati]

Is your feature request related to a problem? Please describe.
Currently, the defaults for all keys are dictated in the RegistrationClass, which can be problematic since there is no "single source of truth".

Describe the solution you'd like
A ConfigManager class, which resides in ereg.configurations would be the appropriate solution. See #59 (comment) for details. The ConfigManager class should read in default parameter values from the sample config, and then we over-write them based on the new one passed by the user.

Describe alternatives you've considered
N.A.

Additional context
N.A.