feat: Add URL capabilities

[beyarkay]

This commit allows the value specified as the first commandline argument to be a URL, in which case it is downloaded (via a blocking reqwest) and parsed into a Calendar struct

I couldn't find many ICS URLs online, but I use them a bunch for my project eskom-calendar, so here are some of those as examples:

╰→ cargo run -- https://github.com/beyarkay/eskom-calendar/releases/download/latest/buffalo-city-block-1.ics
    Finished dev [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/calio 'https://github.com/beyarkay/eskom-calendar/releases/download/latest/buffalo-city-block-1.ics'`

Sun Jun 11 2023
    16:00-18:00 🔌BuffaloCityBlock1 Stage 4 😣
                This event shows that there will be loadshedding on Sunday from 16:00 to Sunday at 18:00 in the load shedding area buffalo-city-block-1.

Mon Jun 12 2023
    21:00-00:00 🔌BuffaloCityBlock1 Stage 3 😟
                This event shows that there will be loadshedding on Monday from 21:00 to Tuesday at 00:00 in the load shedding area buffalo-city-block-1.

Wed Jun 14 2023
    16:00-18:00 🔌BuffaloCityBlock1 Stage 3 😟
                This event shows that there will be loadshedding on Wednesday from 16:00 to Wednesday at 18:00 in the load shedding area buffalo-city-block-1.

    18:00-19:00 ⚠️  End of schedule
                This is the end of the known loadshedding schedule.

[oscarmcm]

Thanks for the contribution!