More infoview tweaks

[Julian]

These are a few other minor possible tweaks for the infoview. Some probably should be defaults, some configurable:

• have one infoview per :tab
• Add a more granular way to control when infoviews autoopen #171
• split the infoview horizontally if there are already two vertical splits on the tab when it is opened
• Allow customizing infoview components via some Lua API #172