add some features:

add some features:
1. display current settings in target.txt by ContextTool
2. display help information with arguments, such as: ContextTool -h, arguments include -h, /h, -?, /?, -help, /help
3. display current setting by ContextTool -x
4. display possible setting by ContextTool -x ?
5. clean current setting by ContextTool -x 0
6. merge new setting to current setting by ContextTool -x new
x is the setting argument, such as p, a, n, m, t, c.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@1746 6f19259b-4bc3-4df7-8a09-765794883524