Add support for multiline strings

[maguro]

• backticks, `, and triple quotes, “””, now allow multiline strings
• split strings into STRING_NAME and STRING_VALUE

[maguro]

This is for "FR: Text proto format could support multi-line strings" and "Add support for multi-line strings".

[kshchepanovskyi]

Thank, I will make sure it is merged and released this week. I'll check code and add some tests, if needed.