Add support for environment variables

[rootCircle]

Motivation

• Currently, because the secrets.py is required to be setup manually, so in absence of that when user invoke codemark --help then some strange "Module" Error occurs, which is irritating in first place.
• Solving it might help devs start working on feature that don't require those env keys at all in future.

Possible Implementation

• Setup up your environment with the instructions in docs/INSTALL.md.
• Then, try moving content of value from hard-coded string to the values in environment variables.
• Ensure not to rename any variable, as it might break the code.

[rootCircle]

One can follow #8, for setting up Firebase environment, before it's merged!

[Kota-Karthik]

@rootCircle I am interested in doing this issue So kindly assign me this!! Thank You : )

[rootCircle]

Sure, go ahead!