Added the ability to plug in a log function

[alexthornton1]

Added the ability to plug in a log function in place of the existing
calls to io:fwrite and error_logger:format. The details are as follows:

(1) Added two new types to unsplit.hrl:

* log_type(), which is either the atom 'normal' or the atom 'error'.
  Each log message has one type or the other.
* log_fun(), which is the kind of fun that writes log messages.  It
  takes two arguments: a log_type() and a string().

(2) Added a key 'log_fun' to the {unsplit, ...} application environment
variable in unsplit.app.src. Its value is a tuple {Module, FunctionName}.

(3) Added a function unsplit:default_log_fun, which is what it sounds like:
the default log_fun when none is explicitly specified. It writes normal
messages using io:fwrite and error messages using error_logger:format,
mirroring previous functionality.

(4) Added three overloads of a function unsplit:log_write, which writes
messages (with or without formatting) to whatever log function has
been configured in the {unsplit, ...} application environment variable.

(5) Added a function unsplit:get_logger, which builds a fun that calls the
appropriate log_fun.

(6) Replaced all explicit calls to io:fwrite(...) with calls to
unsplit:log_write(normal, ...) instead.

(7) Replaced all explicit calls to error_logger:format(...) with calls to
unsplit:log_write(error, ...) instead.

(8) Regenerated unsplit.md documentation file.

When the new 'log_fun' application environment variable has its default
value, unsplit behaves as it did before. However, logging functionality
can now be plugged in to replace the existing functionality.