escape colons in flat subject file export

Resolves #266
Makes the names of the output files of Plugin::Convert::PlainText unique so it doesn't overwrite them.
Merge pull request #275 from QUTlib/feature/subjects-quote-colon

escape colons in flat subject file export
Merge pull request #277 from QUTlib/bugfix/convert-plaintext-multiple-files

Makes the names of the output files of Plugin::Convert::PlainText unique...