prevent dubplicate manifest item ids

this prevents generation of duplicate item ids when manifest item ends in
number. inserting an underscore makes sure that new id's stay unique
(e.g. html2 pos 26 and html22 pos 6 will become html2_26 and html22_6
instead of both html226)

fixes #262
Merge pull request #276 from artm/unduplicate_models

prevent dubplicate manifest item ids