FEAT: Support for +/- to set natures when overriding EVs in Calcdex

[jmynes]

As seen below, Showdown's Team Builder has this feature:

• Assume existing +/- are correct (including blanks)
• If +/- exists and the same symbol is typed, remove from currently buffed/nerfed stat
• Else, Add +/-, adjust nature to match pair accordingly
• If one is present without the other, set to a neutral nature until both are present (maybe indicate this to user?)

This would be great for the Inline EV Editor:

[doshidak]

Hi again LOL.

That's actually a good feature that can be done! I personally do that when I'm in the Teambuilder, but just never crossed my mind about supporting that in the Calcdex for some reason haha.

With the algorithm too, sheeeeeeeesh LOL. Actually spot on to what I would've done, including the neutral nature indicator since we already have something similar when you have unset/unallocated IVs/EVs (colors change). I'll just accept the '+' or '-' character in any IV/EV field so you can quickly just click a field under a stat, hit the + key & be on your way (instead of microflicking to the precise EV field haha).

Thanks for all your great issues!