Add multi-line property file reading

[mattrayner]

• Refactored nextEntry() to check for EOL delimiter
• Added recursive processMultiLine() function which recursively checks for EOL delimiters and concatenates with the next line.
• Added processLine(), moving common line processing into one place

[mattrayner]

I'm working on implementing these changes and improvements now

[mattrayner]

@Rob--W - I've implemented some of your changes, I feel the helper functions greatly improve the readability of the file so have left those in - do you feel I need to take them out?

[Rob--W]

@mattrayner I'm usually favourable towards decomposition of functions, but in this case the original function (nextEntry) is still manageable, while the alternative decomposed functions appear more complex and contain bugs.

PRs should not break backwards compatibility if possible, that's why this method supports synchronous and asynchronous imports.