Expose `flags.cwd` as config option

The chdir logic already comes later in the program so this one should be easy.

[sttk]

@phated I have a question about this flag.

Cli options --cwd and --gulpfile have relation to each other as follows:

$ gulp --cwd aaa/bbb
[23:10:19] Working directory changed to /path/to/aaa/bbb
[23:10:19] Using gulpfile /path/to/aaa/bbb/gulpfile.js
...

$ gulp --gulpfile aaa/bbb/gulpfile.js
[23:11:32] Working directory changed to /path/to/aaa/bbb
[23:11:32] Using gulpfile /path/to/aaa/bbb/gulpfile.js
...

$ gulp --cwd aaa/bbb --gulpfile ccc/ddd/gulpfile.js
[23:35:57] Working directory changed to /path/to/aaa/bbb
[23:35:57] Using gulpfile /path/to/ccc/ddd/gulpfile.js
...

And the current behavior with the config flag.gulpfile is:

$ cat .gulp.json
{
  "flags": {
    "gulpfile": "aaa/bbb/gulpfile.js"
  }
}
$ gulp
[23:13:33] Using gulpfile /path/to/aaa/bbb/gulpfile.js
...

Should these configs flag.gulpfile and flag.cwd have relations like cli options?

[phated]

Due to how complex this issue has become, I’m going to move it to the 3.0 milestone because it might be breaking to some people’s workflows.