Vi input mode: incremental search

[christianparpart]

trafficstars

Based on top of the implementation of #282, being able to incrementally search and highlight would be awesome.

• [ ] enter incremental search mode (/)
• [ ] while entering the search term, incremental search should live-update the viewport and highlighting
• [ ] as soon as a non-insert mode is entered, reserve one bottom for extra input (such as /-search-term).

Depends on: #282.