Allow Breakpoints during task runs

[ferd]

This is mostly useful for tests, where a test suite of any kind can be
interrupted halfway through so that the user can probe the running
system to see what is happening.

This is done as follows:

1. the user must call r3:break() in a test suite
2. the user runs the task as r3:async_do(Task)
3. the test holds up and the user can do whatever
4. the user calls r3:resume() and the test proceeds as normal

A safeguard is added so that breakpoints are only triggered in the shell
in async mode

Sample session:

$ rebar3 shell
...
1> r3:async_do(ct).
ok
...
Running Common Test suites...
%%% rebar_alias_SUITE: .
=== BREAK ===

2> % <do some checks>
2> r3:resume().
ok
3> .....
%%% rebar_as_SUITE: ...........
%%% rebar_compile_SUITE: ......
...