Changed "end-of-string" string

Earlier pico-wf used the line with "===" string as end of string indicator.
I thought if one day we add support for markdown, that would be a problem,
because "===" can be used to mark the header. Now the string terminating
line is "===EOS===", which should not conflict with anything.
Merge branch 'feature-better-string-terminator' into develop