Document the newly added "OR" operator

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Document the default output format

The default output format is PLAIN.  This format is a bit problematic, as it
cannot be unambiguously parsed, so it will probably be replaced in the future.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Use native LLVM types during expression evaluation

This is a major refactoring, which significantly shortened both the C++ code
and the generated code.  Other important changes include:

  * Strings are formatted directly into the output buffer, rather than in a
    temporary buffer
  * More code is shared between JSON and CSV formatters
  * The PLAIN format goes away, and CSV is the new default
  * The current date format string and output format enum are turned into
    global variables

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Added support for more comparison operators: < <= >= > !=

These operators work both for strings and for integer values.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>