Fix LaTeX "no line here to end" problem with newlines within text commands

```latex
\textbf{foo\n\nbar}  % LaTeX Error: Paragraph ended before \text@command was complete.
\textbf{foo\\\\bar}  % LaTeX Error: There's no line here to end.
\textbf{foo\\\vspace{\baselineskip}bar} % works, but is a hack
```

This change makes line breaks in the text commands be converted to vspace's as in the last example.