Expose user agent option to DOM navigator interface.
layout: Make content of `display: inline-block; overflow: hidden` visible.
auto merge of #3608 : glennw/servo/user-agent-navigator, r=Ms2ger

This also makes command line options available as a global. If we're happy with that change I will go through the rest of the code and update it to avoid passing and cloning the Opts structure.
auto merge of #3604 : pcwalton/servo/inline-block-overflow-hidden, r=glennw

Makes lots of GitHub appear.

r? @glennw
layout: Refactor inline layout to remove the code that tried to avoid
splitting fragments.

I don't think it will be possible to avoid splitting fragments in the
presence of `vertical-align`, because one `ScannedTextFragment` could
potentially be split into arbitrary many fragments, each having its own
vertical position that can influence layout of other fragments.

This code also removes parts of `Range` that were no longer used.