Added a few features from Vimperator

Added three new commands:
* `Vomnibar.activateWithCurrentUrl`: opens the vomnibar, but pre-populates it with the current URL. Especially useful if you want to navigate to a slightly different URL path. I tend to map `O` to this, and `o` to Vomnibar.activate
* `Vomnibar.activateWithCurrentUrlInNewTab`: the same as above, but opens the resulting URL in a new tab. I usually map `t` to Vomnibar.activateInNewTab, and `T` to this
* `removeTabAndGoLeft`: the current removeTab command closes the current tab, and leaves the user on the tab to the right of where they were. This just does the opposite. As an example, I map `d` to removeTab and `D` to removeTabAndGoLeft