Remove the --cwd option and introduce smarter cwd-resolver

[pomek]

At this moment, a user can provide the --cwd value. This does not make sense.

The cwd should indicate on the directory which contains the configuration (mgit.json) file.

cwd-resolver module looks like:

module.exports = function cwdResolver() {
	return process.cwd();
};

It should be smarter and finds the mgit.json file. If the file was not found then goes one level up and tries to find the file once again. After n attempts, it should throw an error - "The configuration file cannot be found." or something like that.

[Reinmar]

The cwd option was already removed. Now we need a smarter resolver which scans dir tree up for mgit.json or package.json (once we'll support configuration through package.json).

[Reinmar]

DUP reported in #71

cc @adelura