Default configuration file can be overriden via BTEST_CFG environment
variable.
Adding CommandPrefix option.

CommandPrefix
    Changes the naming of all btest commands by replacing the
    "@TEST-" prefix with a custom string. For example, with
    "CommandPrefix=$TEST-", the @TEST-EXEC command now becomes
    "$TEST-EXEC".
Adding Initializer option, which runs a command before each test.
Documentation updates.
Smaller fixes and tweaks.

Includes:
    - When creating directories, we know also create intermediaries.
      That in particular means that "@TEST-START-FILE a/b/c" now
      creates a directory "a/b" automatically and puts the file in
      there.

    - IgnoreDirs now also works for sub directories.
A test-suite for btest. Using, of course, btest.

"make test" will test most of btest's features. The main missing piece
is testing the Sphinx support, we will add that next.