LaTeX reader:  handle leading/trailing spaces in emph better.

`\emph{ hi }` gets parsed as `[Space, Emph [Str "hi"], Space]`
so that we don't get things like `* hi *` in markdown output.

Also applies to textbf and some other constructions.

Closes #1146.  (`--normalize` isn't touched by this, but
normalization should not generally be necessary with the
changes to the readers.)