async callable objects

With the following script:

```
class Build:
async def __call__(self):
pass

import fire
fire.Fire(dict(build=Build()))
```

Doing `./test.py build` outputs:

```
NAME
test.py build

SYNOPSIS
test.py build GROUP | COMMAND | VALUE

GROUPS
GROUP is one of the following:

cr_code
code(argcount, posonlyargcount, kwonlyargcount, nlocals, stacksize, flags, codestring, constants, names, varnames, filename, name, firstlineno, lnotab[, freevars[, ce$

cr_frame

COMMANDS
COMMAND is one of the following:

close
close() -> raise GeneratorExit inside coroutine.

send
send(arg) -> send 'arg' into coroutine, return next iterated value or raise StopIteration.

throw
throw(typ[,val[,tb]]) -> raise exception in coroutine, return next iterated value or raise StopIteration.

VALUES
VALUE is one of the following:

cr_await

cr_origin

cr_running
```

Instead of firing `Build().__call__` as expected.