hyphen adds double space

[fersarr]

Hi, I think support for negative numbers is not yet here, but the hyphens seems to be messing the string up and adding an extra space. Might be related to #29 :

>>> t2d.convert('he wanted to know what is three - negative two')
'he wanted to know what is 3   negative 2'

Note the double space!

expected:
'he wanted to know what is 3 - -2'
or without negative number support:
'he wanted to know what is 3 - negative 2'

It seems the hyphens are messing it up since it works okay-ish with '+':

>>> t2d.convert('he wanted to know what is three + negative two')
'he wanted to know what is 3 + negative 2'