Support negative integer literals in types

[23Skidoo]

Fixes #19.

Patch by @wojtekmach.

[23Skidoo]

I have tested that this patch solves the problem for me:

iex(2)> Spect.to_spec(Decimal.new("1.23"), Decimal)
{:ok, #Decimal<1.23>}
iex(3)> Spect.to_spec(Decimal.new("-1.23"), Decimal)
{:ok, #Decimal<-1.23>}