Panic recovery

Adds new config option to recover panics caused by command handlers.

Used by either:

• setting BOTSU_NO_PANIC env variable to a truthy value (1, true, t, etc.)
• setting no_panic in the TOML config file to true

When enabled, panics rooting from handlers will be logged as errors along with the stack trace. If not enabled, the program will crash.

Full Changelog: v0.2.1...v0.2.2