Fix exporting values which contain equals character

[agologan]

Whenever the value in a Twine file contains an equals sign the line split will truncate the actual value.

For e.g. when using xliff in Android values the following line in a Twine file: en = Learn more at <xliff:g id="prod_gamegroup">Game Group</xliff:g> will be split twice resulting in the value being: Learn more at <xliff:g id

To fix this we're adding a field limit of 2 when splitting the string, the resulting split being done only by the first equals sign.

[dasheck0]

Thanks for this pull request. I was aware that this might be a much needed feature but until now I never came across the need. I will gladly merge it.