Fix #11

The `=` sign is required because it's not inside the square brackets:

[bin/stackprof#L16](https://github.com/tmm1/stackprof/blob/master/bin/stackprof#L16)
```ruby
o.on('--limit=[num]', ... )
```

[bin/stackprof#L18](https://github.com/tmm1/stackprof/blob/master/bin/stackprof#L18)
```ruby
o.on('--method=[grep]', ... )
```

[bin/stackprof#L19](https://github.com/tmm1/stackprof/blob/master/bin/stackprof#L19)
```ruby
o.on('--file=[grep]', ... )
```

This fixes the bug.