Consider defining a mode for async-shell commands

[sebasmonia]

dotnet clean runs in one (no point in using compile for it) and it would be nice:

1. if the output was read only
2. if q would kill the output buffer
3. start running command and finishing showed custom messages in the echo area

Only worth it if there were a few other commands that use async-shell-command.

[sebasmonia]

One thing that I need to fix is that for some commands if I want to move point to the beginning of the buffer (for example for the info buffers) it doesn't work with the current implementation.

[sebasmonia]

After solving #13, the mode could take care of making things read-only etc. The only other proper command using async-shell-command is dotnet publish.