[JSON] strings can't span multiple lines, fix comment block detection

[keith-hall]

• strings are not allowed to contain new line characters, so mark them as unterminated and pop from the string context when a new line is reached. See http://stackoverflow.com/a/16690186/4473405 for clarification
• update comment block detection to match the JavaScript syntax and fix an empty block comment not being seen as closed
• scope dictionary keys so they can be targeted by color schemes Scope for keys in mappings/dictionaries (e.g. JSON, YAML) #421