Backport PR #4125: Basic exercise of `ipython [subcommand] -h` and help-all

for various subcommands.

Prompted by the fact that `ip qtconsole -h` and `ip nbconvert -h` are both broken in master.
This PR also fixes both.

Should be backported to 1.1
Backport PR #4141: add files with a separate `add` call in backport_pr

`git commit [filename]` will fail if the filename is not already tracked.

/cc @ivanov