Allow extra command line options for `pdal pipeline`

This patch adds the ability to do `pdal pipeline pipeline.xml --drivers.las.writer.filename=output.las`. Note that this example only exhibits defined behavior when the original pipeline does not have a filename option — if the original pipeline does have a filename option, we can't be sure which filename will come first, since we have equal keys in the multimap. Suggestions for improving this behavior are welcome.

This pull request also includes a commit that defers option processing until `processOptions` in the las writer.

The actual [extra options handling](https://github.com/gadomski/PDAL/compare/PDAL:master...gadomski:pipeline-commandline-options?expand=1#diff-0698b647651b6618f57fb7e13313883aR106) was copied straight from `pdal translate`, and could potentially be refactored out to a utility method, though I wasn't sure if it was worth the added complexity.