Added new flags for show cards and added show comments

[whitef0x0]

This PR adds the following features

1. For show cards
   The "assigned" flag, --assigned|-a, when enabled only shows cards that are assigned to current user for the current board.

The "completed" flag, --completed|-c, when enabled only shows cards that are assigned to current user and are in the "done" list for the current board.

The "include_closed" flag, --include_closed, when enabled shows cards that are both non-archived (open) and archived (closed). This changes the default behaviour of cards to only show non-archived cards when the "include_closed" flag is not specified

2. For show unresponded
   This command shows all unresponded comments that mention the current user's username. This is not restricted to the current board and will fetch comments from all boards that the user is part of.

[delucks]

Hey @whitef0x0! Thanks for opening a PR, I appreciate your contribution to the project. I will review your code changes the next time I have some focus time to thoroughly look through them. I am excited to try out the new commands and option flags you added.

One quick comment- I noticed you renamed the tool from gtd.py to mello. Could you back out the renaming changes in this PR and suggest the change of name via a new issue? I'd like to get buy-in from users before we make such a large change to the user interface and location of the project.

[whitef0x0]

@delucks yup I'l change it back asap