Overhaul command line argument processing to use clap instead of
gumdrop.

Need this to resolve the dependency conflict on clap.
Does not compile because of the API break.

The version string for chain config memos that was previously obtained
from abscissa is obtained with the crate_version! macro from clap.

Tests rely on the `version` subcommand being implemented.

This is cumbersome, but it's the only way to support multiple
occurrences of --event flag as of clap 3.0.0-beta.5.
Should be fixed by the clap 3.0.0 release.

To avoid a panic in the terminal component initialization over
an attempt to install a global eyre handler after the application has
already done so, we need to tell the terminal to not use colors.

To avoid confusion with the --version flag conventionally used to print
the program version, the new long name is --channel-version.
The --version flag is still supported with the present meaning on the
`create channel` subcommand, but this alias is no longer described
in built-in help.