Option to control entity ID prefix in standoff output

[spyysalo]

NERsuite standoff output (-o standoff) currently uses the ID format entity-NUM, where NUM is some positive integer. It would make it easier to integrate NERsuite into some processing pipelines if it were possible to set the entity ID "prefix" (i.e. entity- at the moment) with a command-line option.

[priancho]

Ok, I will add some options to control the output template in the standoff output mode.