lobster-online-report shall not modify the input, but generate new output, to support bazel

[phiwuu]

Bazel requires that there is always an input and an output to a tool, so that Bazel can use smart caching mechanisms. Unfortunately lobster-online-report modifies the input file.

Please add a new command line flag --out that allows to specify the output file. If no value is given, then the input file is modified (for backward compatibility).