LaTeX reader:  Better error messages with include files.

Closes #1274.

Rewrote handleIncludes.

We now report the actual source file and position where the error
occurs, even if it is included.  We do this by inserting special
commands, `\PandocStartInclude` and `\PandocEndInclude`, that encode
this information in the preprocessing phase.

Also generalized the types of a couple functions from
`Text.Pandoc.Parsing`.