add function for messing up the separators in strings

[luisDVA]

addresses #12

added the alter_separators function in the same style as the existing fns, with corresponding tests and participation in messy(). Let me know what you think.
cheers

[nrennie]

Thanks for contributing this @luisDVA! I've made a few edits before merging:

• I've renamed the function to use change instead of alter to keep it consistent with existing functions.
• Made the code slightly more modular.
• Removed some of the tests that have incorrect assumptions (the messiness parameter is probabilistic, not deterministic).