Superscript power characters support is too limited

[Tey']

SpeedCrunch is able to understand powers when using superscript characters like x³, but it only works if the power is a positive integer between 0 and 9. Thus, copying expressions that contain negative powers like x⁻² or multi-digits powers like x¹² from documents, and pasting them into SpeedCrunch do not work (the latter being converted to x^1^2 which is a bug).

Also, supporting negative integers permits to format units like meter / second in a prettier way (meter second⁻¹ vs meter second^-1).

[Tey']

Should be fixed/implemented in pull request #55.