[plan-build] Add limited readline functionality to `attach()` function.
[plan-build] Add limited readline functionality to `attach()` function.

This change enables minimal command history and file tab complete for
the `attach` (i.e. "debugger") helper. It's pretty sweet.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,nathenharvey/habitat,rsertelon/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,georgemarshall/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,rsertelon/habitat