Fix: "ti clean" does not scan SDK hooks

[infosia]

Fix for TIMOB-20085

Currently there's no chance for Titanium SDK to load "hooks" while ti clean even when you subscribe clean.post event. It is required from Windows SDK to load hooks before ti clean because it uses special directory for Visual Studio.

This PR basically gives platform-SDK a chance to load hooks before ti clean, via scanHooks function in _clean.js. platform-SDK that needs clean-hook should scan hooks in cli/commands/_clean.js, and then subscribe clean.pre, clean.post and so on.

_cli/commands/clean.js

var path = require('path');

exports.scanHooks = function(logger, config, cli) {
    cli.scanHooks(path.join(__dirname, '..', 'hooks'));
}

cli/hooks/clean.js

 exports.init = function (logger, config, cli) {
    cli.on('clean.post', {
        post: function (builder, finished) {
... // platform-specific clean-up
            finished();
        }
    });
};

See discussion in https://github.com/appcelerator/titanium_mobile_windows/pull/505 for more details.

[sgtcoolguy]

Do we really need to look for a platform-specific cli/commands/_clean.js, load that and call scanHooks on it? Shouldn't we just call:

cli.scanHooks(path.join(__dirname, '..', '..', platform, 'cli', 'hooks'));

Basically just say load all hooks from the platform specific cli hooks directory.

[infosia]

Yeah, I just wanted to make it flexible for platform SDK to choose hook directory but I might be thinking too much.