texcount: add -sub example

[YDX-2147483647]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known): TeXcount version 3.1.1, 2018 Oct 28.

-sub can be expanded to -subcount, but I think -sub is clear enough, so I write -sub.

[kbdharun]

LGTM, thanks for your contribution.