Support for additional arguments for browser tests

[zoechi]

examples
content_shell --dump-render-tree some_test.html
chrome --allow-file-access-from-file some_test.html
Even better would be have dart-lang/test#23

[seaneagan]

I'm hoping to add task-specific command-line args, see #116.

I assume you will prefer to use the unittest runner via a grinder test task or similar, in which case it depends how that accepts these options, I wonder if they can be in the config file.

[zoechi]

Package level config file is probably to coarse-grained. I hope they will add additional configuration options for at list single test files. If the task-runner from the unittest package is used this issue might be redundant. I hope there will be a good integration.

[zoechi]

I think this can be closed. This is something the test package should provide.