Keep control spaces

[masonproffitt]

Resolves #117.

Retain control spaces ('\ '), as they change the meaning of the code. For example, some initially valid TeX:

>>> TexSoup.TexSoup(r'\ foo')
\foo

becomes an undefined control sequence. With this PR, you get:

>>> TexSoup.TexSoup(r'\ foo')
\ foo

[coveralls]

Coverage remained the same at 100.0% when pulling 8d68a717e4cb3876f2bb269ddf781224f6719a71 on masonproffitt:fix_117_retain-control-spaces into f91d4e71b21aa6852378d2d60ecc551b39e05bf0 on alvinwan:master.

[alvinwan]

Thanks @masonproffitt! Sorry it took so long to get around to these. Much appreciated 🙏🏻