sliceOn (Based on #16)

Allows users to define a character/string to slice before if it is found in the summaryText. Although it works for anything, it can (and I imagine will) be used for line breaks. Set up with tests to check functionality and behavior.

NOTE: It will avoid creating a slice on the set character if it is located inside HTML tags, as it abides by the exact same rules governing normal slice points with expander.

Closes #16, 
Resolves #59