Support flags as options in .gulprc file

[phated]

The flags should be able to be defaulted in the .gulprc file so people can rely on gulpfiles running in a certain way. e.g. running with a certain log level

[phated]

@sttk if you plan to implement more features in .gulp.*, start with this one before attempting #57 or #63

[sttk]

@phated Since the configs are merged with opts, the flags are already able to be configured with .gulp.* files.

But those values aren't checked at all. Are the checks needed and are there any flags forbidden to configure with .gulp.* files?

[phated]

@sttk oh, I don't think that's the way I wanted it to work. flags should be a property that gets merged into the flag options. How should we handle this?

[phated]

Basic support for this was completed by #92
@sttk does that PR add support for every flag or just the ones specified? If it only supports a subset, should we leave this open?

[sttk]

@phated I think #92 is for creating mechanism to support flags.
We may add other flags or configs later as needs or requests.
So this issue might be closed.

[sttk]

@phated Or are there any flags what you are already thinking that we should add?
I'll implement to support those flags if you list them.

[phated]

I think I've opened issues for the 3 remaining flags that make sense as config options. See #108 #109 and #110