Fix space-separated single character commands

[lluchs]

These commits fix the handling of single-character commands with a letter.

First, \b would usually not produce the correct output, since there are no composed characters with an underline in Unicode.

Second, these commands were only supported with a brace argument, like \c{c}. However, TeX also accepts these with the argument separated with a space, like \c c. This form comes up in the output of zotero-better-bibtex.