Pause execution and return to CLI on ^C

Consider this a feature request with a prototype, I guess, because I get the following when I actually try to use this:

```
dlv> continue
new thread spawned 12773
new thread spawned 12774
^CStopped at: :269
Command failed: open : no such file or directory
dlv> 
```

Is this just what happens when it stops in the runtime guts rather than go-land?
I would move this logic down to after the switch where we try all of the different ways to attach to a process. The start() local function was removed by #12 so be sure to rebase on top of master to pull in changes to main().

Also, the start() function is only run when executing a binary on behalf of the user, so in the case of attaching to an already running process this signal handler would never get registered. Putting this at the end of the switch in main will ensure this signal handler is registered regardless of how we attach to the process.