update the connection code to be flexible on the prompt characters

[RobertBlackhart]

Fixes #30

The code will now look for 3 possible prompt values:

• The >>> standard MicroPython prompt
• The --> aiorepl prompt
• Any custom prompt the user has set on sys.ps1

Here's a connection to a board with no customized prompt (same behavior as before the change):

Here's a connection to a board that has the aiorepl demo script in its main.py:

If ViperIDE detects the user has a custom prompt, it will log that fact: