Use file extension “txt” for compiled built-in help files.

The previous extension “adoc” is used for ASCIIdoc markup language. “txt” is
more appropriate because this is plain text.

Allow for emphasis in .doc files by using *-delimiters

Adjust .doc processors to die with more information

In Poly/ML at least, you need to explicitly catch and do something with
exceptions. Otherwise, the process quietly dies with an error exit code.

Don't use fancy UTF8 quotes when converting text help files on Windows.

I did look into the possibility of changing the help system to
dynamically convert quotes when coming to display the file (if the
Unicode flag was off, you could convert the fancy quotes back to pure
ASCII as the file was displayed). But there doesn't seem to be a
good/easy way of getting into the internals of the Moscow ML's help
system's display routines for this functionality. And of course, it's
people using Moscow ML on Windows that are most likely to have trouble
displaying the fancy quotes. So, this is a somewhat hackish way out.
We can revisit if and when we drop support for Moscow ML.

Closes #40.

Make "text" docfiles use Unicode quotes.

This is not ideal for people with Unicode off.
Progress with #40.

Got the help build system working on PolyML.

Start working on porting the doc building system to SML (i.e., not Moscow ML)
to get it working with Poly ML.