`tarbell generate` requires directory argument, docs says it's optional

[jheasly]

In 1.0.4, if you run tarbell generate without specifying a directory you get:

$ tarbell generate

You must specify an output directory (e.g. `tarbell generate _out`)

This contradicts the docs, which say the output directory's optional.

I think it's a logic issue in cli.py.

The 1.0.4 docs say if no directory is supplied, a temporary one will be created, which gets done here, based on output_root evaluating to False.

But if I'm reading the code right, if output_root is False, you'll never get down to that line as the else here traps for output_root evaluating to False.

[eyeseast]

Reopening per discussion on #403. Turns out I missed a page.