Telnet: add parameter to allow stripping the prompt

The "Read Until Prompt" and "Execute Command" keywords return the output
and including the prompt. This patch adds a new parameter "strip_prompt" to
these keywords which if set to true strips the prompt from the output.

Fixes #1874.
I meant at the end of the paragraph. But separate comment like at the end of the description is fine too. It's just important that this is mentioned somewhere.

Because `Execute Keyword` refers to `Read Until Prompt` I don't think it needs to have this note. Obviously it doesn't hurt either.