c++-mode/cout: improve

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.
Merge pull request #62 from YorkZ/pull-request

c++-mode/cout: improve