c++-mode/cout: improve

Hi,

I've improve the "cout" snippet. Below is a copy from the commit message.

Improve this snippet by:

1. Using '\n' instead of "std::endl" to improve the speed of streaming output. Generally, "std::endl" should be used only when absolutely necessary because sending "std::endl" flushes the stream which is an expensive operation.

2. Giving user a chance to decide whether to keep the name space prefix "std::" (type "C-d" to delete the name space prefix).

3. Giving user a chance to decide whether to keep the entire part of " << '\n'". If kept, she can further decided whether to change the '\n' to something else.

York