Added --stdout flag to output single file transforms to stdout

To use this tool in combination with others, I found it useful to have a option which allows to pipe the output of  a single file transform to stdout instead a file.

Changes:
* Added `output` variable, which can be either file or stdout
* Added `log()` and `logError()` functions so that no debugging messages are written to stdout if `output` is set to `stdout`

Example Usage:

    doctoc README.md --stdout