Allow for reading from stdin and writing to stdout.

`addlicense` currently always modifies source files in-place. This makes it difficult to use in scripts or integrate into other formatting tools, such as the [`fix`](https://www.mercurial-scm.org/repo/hg/file/tip/hgext/fix.py) Mercurial extension.

Since the filename is unknown when operating on content from stdin, this would entail adding an option to specify the filename to assume. For example, `clang-format` has an `--assume-filename` option for this purpose.