Add option to influence the calculation of derived change requests and features

[aschaal]

.. requirement:: CRS_1
    :category: input
    :change_request: a, b
    :refs: SRS_1

.. requirement:: CRS_2
    :category: input
    :change_request: b, c
    :refs: SRS_1

.. requirement:: CRS_3
    :category: input
    :change_request: a
    :refs: SRS_1

.. requirement:: CRS_4
    :category: input
    :change_request: a, b
    :refs: SRS_1

.. requirement:: SRS_3
    :category: software

SRS is current generated to: Derived Change Request: a, b, b, c, a

The reason is, that only the complete change_request strings are compared when merging.
Merge options could be

• a list of substrings to remove in general
• a list of separators like ","
• compare case (in)sensitive
• ...

Needs to be done for the feature field as well.