Enhancement: Log to stdout using the `--log -` option

Logging to stdout was possible using `--log /dev/stdout`. However,
when the rest server is run as a different user, for example, using

	`sudo -u restic rest-server [...] --log /dev/stdout`

this did not work due to permission issues.

For logging to stdout, the `--log` option now supports the special
filename `-` which also works in these cases.

https://github.com/restic/rest-server/pull/217