Add hh_client option to show function type

[steelbrain]

Currently we have to use a hack suggested by @SiebelsTim, which is to use AutoComplete to show function arguments. It'd be nice to have it built-in to it.

[jwatzman]

Several people have asked for the ability to, given a function/method name, show its arguments and return type. hh --show can dump this in a human format already (but tools should emphatically not use that), autocomplete can do it in terms of completions, but would be very useful to do just given a fixed name.