Adding functionality to limit the number of characters from a variable

[joemar1988]

(anything else with substring) Any other argument in the following format ${ARGUMENT, "n"} or ${ARGUMENT, "-n"} where ARGUMENT is your environment variable and n is a whole number. This can be used to limit the size of the environment variable you are using, taking the first "n" characters or the last "-n" characters from the variable.

[jenkinsadmin]

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[DenizThatMenace]

This seems to work, although it took me some time to figure out, how and where exactly this should be used.

This brings me to two things that are missing before this pull-request can be accepted.

1. Some help/documentation.
   Probably you should extend the help-message for the Version Number Format String text-field.
2. Tests
   Please add a test which explicitly tests the different cases. Not only test the ones that are expected to work but also those that are expected to fail.

Just as a side-note:
Should ${VARIABLE, "+5"} do the same as ${VARIABLE, "5"}? (It does not.)

[joemar1988]

Thanks for letting me know. I will get these updates in as soon as possible.