Revamp tz command

[tk3369]

The current tz command is somewhat basic. Some of the shortcomings are:

• It can only convert to a set of standard time zones (aliases)
• Time zone aliases can only be changed via PRs
• It cannot convert to a specific time zone that is not in the standard list.

I would like to revamp the interface as follows:

Examples

Update user preference

Use the “me” sub-command to register user preference. The data is stored and remembered so the user just needs to set it up once.

> tz me
Hey @tk3369, type `tz me [timezone]` to set up your local time zone. 
You can find a list of time zones at https://en.wikipedia.org/wiki/List_of_tz_database_time_zones

> tz me America/Los_Angeles
Hey @tk3369, your local time zone has been updated to America/Los_Angeles.

> tz me America/wat
Hey @tk3369, sorry, time zone "America/wat" is not valid. 
You can find a list of time zones at https://en.wikipedia.org/wiki/List_of_tz_database_time_zones

Set up system-wide aliases

Use “alias” sub-command to list, add, or remove time zone aliases. List results are sorted by alias names.

> tz alias list 
Hey @tk3369, the time zone aliases are:
- BRU: Europe/Brussels
- LA: America/Los_Angeles

> tz alias set NY America/New_York
Hey @tk3369, a new time zone alias "NY" has been set to "America/New_York"

> tz alias rm NY 
Hey @tk3369, time zone alias "NY" has been removed.

> tz alias rm BMW
Hey @tk3369, sorry, time zone alias "BMW" does not exist.

> tz alias set modtz America/Los_Angeles, America/New_York, Asia/Kolkata
Hey @tk3369, the new alias group `modtz` has been added with the following time zones:
- America/Los_Angeles
- America/New_York

> tz alias list
Hey @tk3369, time zone aliases are as follows:

- BRU: Europe/Brussels
- LA: America/Los_Angeles
- modtz: America/Los_Angeles, America/New_York, Asia/Kolkata

Convert to MY timezone

> tz convert 2021-03-21 03:00PM NY
Hey @tk3369, the local time for 2021-03-21 03:00PM NY is 2021-03-21 12:00PM America/Los_Angeles.

If I haven't configured my local timezone, then:

> tz convert 2021-03-21 03:00PM NY
Hey @tk3369, you have not configured your local time zone. Type `tz me` command to set that up.

Convert to OTHER timezones

Add a “to” function. Display results in standard time zones.

> tz convert 2021-03-21 03:00PM LA to NY
Hey @tk3369, 2021-03-21 03:00PM LA is same as 2021-03-21 06:00PM NY

> tz convert 2021-03-21 03:00PM LA to BMW
Hey @tk3369, sorry, time zone "BMW" does not exist.

> tz convert 2021-03-21 03:00PM LA to modtz
Hey @tk3369, 2021-03-21 03:00PM LA is the same as:
- 2021-03-21 03:00PM Americas/Los_Angeles (LA)
- 2021-03-21 06:00PM Americas/New_York (NY)
- 2021-03-21 03:30AM Asia/Kolkata (India)