Robotframework-debuglibrary

* add `step`, `next`, `continue`, `list` and `longlist` commands

* drop python 2 and 3.4 support
* upgrade prompt-toolkit to 2.x
* prompt-toolkit 3 is not compatible with python3.5
* split the huge source file into small pieces
* refactory

* change the minimum requirement of robotframework to version 2.9
* support keyword document including non-ASCII characters
* support import library with arguments
* support passing pybot's arguments, disabled all logs by default
* support press ctrl+c to interrupt running keywords
* support comment format of robotframework
* print value if input variable name only

* use python-prompt-toolkit to handle prompt with color support
* auto completion for robotframework keywords and debug shell commands
* save history to file
* rename shell script rfshell to rfdebug