feat: -t flag for absolute time

[ravener]

Implements the ability to pass an absolute time.

Notes:

• I don't particularly like the brute-force time parsing done here but I'm not sure if there's a better way, open to suggestions.
• Spaces in the AM/PM like '9 PM' would be subject to shell parsing (i.e they'd be split to two arguments and won't work unless quoted) not sure if that impacts the user experience.
• I made -t a string flag, hence had to make the main positional argument optional, but after that I remembered it's also possible to make -t a boolean flag and re-use the positional argument, simplifying the cli arg parsing logic, but then I'm unsure if that's a good experience for using flags since I feel like you'd expect the flag to take an argument, would love to hear your take on this.

Lemme know if you want anything to be changed.

Closes #6

[ravener]

Didn't realize that #156 already exists, oops, feel free to close this if you prefer that one.