export: Don't require log directory to exist

`-l`/`--log` is the directory to place process logs in, so you should
only need this directory at runtime. The log directory shouldn't be
required to exist at export-time, because you might just export the
Procfile to a supervisord config and then use that exported config to
run processes on another box altogether.

[marca@marca-mac2 export_test]$ ls -l
total 8
-rw-r--r--+ 1 marca  staff  23 Nov 26 08:15 Procfile
[marca@marca-mac2 export_test]$ honcho export . supervisord
[marca@marca-mac2 export_test]$ ls -l
total 16
-rw-r--r--+ 1 marca  staff   23 Nov 26 08:15 Procfile
-rw-r--r--+ 1 marca  staff  354 Nov 26 11:15 export_test.conf

Fixes: GH-90
Merge pull request #94 from msabramo/export_no_require_log_dir

export: Don't require log directory to exist