Calculate instance values for com.typemytype.robofont.italicSlantOffset #59

[LettError]

Create a mutator for values in com.typemytype.robofont.italicSlantOffset. Calculate the value when making an instance UFO.

• getItalicSlantOffsetMutator() collects the values from the relevant sources.
• only add the value to the instance UFO lib if it is not zero. Not entirely sure about this, but it seems cleaner?
• Lib values added to the ds5 test fonts.

[LettError]

This will look for lib keys in doc.libKeysForProcessing and try to process them. italicSlantOffset is preloaded to the list. The test data has an extra key and values added and the values appear in the instances.