Subcommands for different steps

[langston-barrett]

Icemelter is growing into a multi-step process:

• Grab the MCVE from the issue (Download ICEs from Github issues #4)
• Find how it produces an ICE (Try different compilers/flags to achieve ICE #18)
• Reduce it with treereduce
• Try reformatting it (Run rustfmt iff it maintains the ICE #11, Strip leading/trailing newlines from reduced programs #17)
• Bisect it (Bisect with cargo-bisect-rustc #16)

Many of these are reasonable things to want to try on their own. Icemelter should expose a subcommand for each of these, and a melt subcommand that does them all in a row.