Implement CLI for date conversion

[aj3sh]

Let's implement CLI for date conversion, English to Nepali (AD to BS), and Nepali to English (BS to AD).

An example of a CLI would be:

ad2bs 2023-04-29
# Output: 2080-01-16

bs2ad 2080-01-16
# Output: 2023-04-29

We can also decorate the output, with something like the below:

Nepali Date:
Saturday, Baishak 16, 2080
शनिबार, बैशाख १६, २०८०

English Date:
Saturday, April 29, 2023