web: allow outdir to be set when running --web mode

[carbocation]

In dream.py, the outdir is created whether run as a command line app or a web app.

However, the outdir is only respected if run as a command line app. It would be nice if --outdir were respected from the web app as well.

[thealanle]

I submitted a fix (PR #373), which gets dream_web_log.txt and output and intermediate images working with --outdir in the web app. Note that the --prompt_as_dir argument doesn't work with the web app yet.