help, the pager, and keyboard shortcuts

currently calling `help(some_function)` in the R and Python kernels outputs a helppage via normal output mechanisms (inline), which doesn’t fit well to the usual quick-and-volatile access to help pages users want.

it woule be awesome to fix #1135, and introduce a keyboard shortcut (e.g. F1) which sends a help request. help requests are defined that they receive a variable name or string and return a mimebundle which then is displayed in the pager.