[opt-viewer] opt-viewer.py takes -o argument

Summary:
Change how the output directory is specified when invoking
opt-viewer.py, from `opt-viewer.py yaml_file_one yaml_file_two output_dir` to
`opt-viewer.py -o output_dir yaml_file_one yaml_file_two`.

This makes it easier to pipe the results of another command into
opt-viewer.py. For example:

```
find . -name "*.yaml" -print | xargs /path/to/opt-viewer.py -o html
```

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D34711

llvm-svn: 306694