Fix unit detection for negative values

The regular expression used to extract the unit (%, em, ...) doesn't work with negative values (and plus-prefixed values, but that's just an edge case).

Adding a `?` after the equal sign allows Morpheus to work correctly when animating to values such as -100%, -50px, -2cm and +150%.

A very common use case is writing carousels / page sliders et similia since we often want to animate left to -100%.