support negative numbers in eskip

[aryszka]

:)

[LappleApple]

Hi @aryszka, you still planning to do this? Suitable for "Help Wanted" label?

[aryszka]

Hi @LappleApple , this is kind of an embarassing issue 😆 ... we are planning to create a better parser and a somewhat extended syntax for eskip. See #182

Since it seems like it's taking a while, this one can be an easy one for someone who takes it. I add the helpwanted label.

[LappleApple]

Thanks, @aryszka.