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.
Needs a note at the end like

    Optionally stripping prompt is a new feature in Robot Framework 2.8.7.