Fix in tools/anntoconll.py to handle cases where multiple spans of text are covered by a single entity

[ssubraveti]

In brat ann files, sometimes, multiple spans of text are covered by a single entity. These are formatted by using a ;, for example:

T1	E1 77 128	some text
T2	E2 7 75	some other text
T3	E3 148 346;347 354	some other some other text

The tool that converts ann files to CONLL format fails to handle this case.