Adds invalid string errors

Beefs up the mt_error handler with invalid string errors. These all take
place in the construction of the note (the only place in the parser
where strings are set). As notes are immutable, with respect to string
and fret numbers, the error checking only needs to take place in the
constructors.
Small doc tweaks to MtString
Adds handles for each conf id

For 1.0, I am only targeting the basic configuration necessary to make
MarkTab useful. The conf values included are as follows:

* tuning: The tuning for each string. This also inherently sets the number
of strings.
Default - E A D G B E

* line_length: The length of each line in the output.
Default - 80