Allow specifying a kernel installation directory with the installer

[SpencerPark]

A request was made in SpencerPark/IJava#43 for an installation script that supports specifying an absolute path to install the kernel to. Although this may result in installations that are not visible to jupyter, it seems useful enough to warrant support.

With the python installer I would expect one could write python3 install.py /kernel/install/path which would copy the kernel files to /kernel/install/path/<kernel_name> and update the kernel.json accordingly.

Depending on how easy it would be to detect, we could also display a warning if the installation directory is not visible to jupyter.

[SpencerPark]

This was misinterpreted and is actually a lot more challenging than initially expected as we are using python's kernel installer which doesn't appear to support this. The request was to run the script from anywhere and this was added in 577a548.