How might it be possible to add IJulia to `KNOWN_KERNELS`?

As far as I can tell, the value of argv in a kernel.json corresponding to an IJulia installation should be:

["/path/to/binary/julia", "-i", "--startup-file=yes", "--color=yes", "/path/to/install/location/of/IJulia/kernel.jl", "{connection_file}"]

Comparing this to what's already in envkernel.py, it seems like adding IJulia as a "known kernel" would amount to something similar to:

KNOWN_KERNELS = {
...
},
'ijulia' : {
'language' : 'julia',
'argv' : ['julia',
           '-i',
           '--startup-file=yes',
           '--color=yes',
           'kernel.jl',
           '{connection_file}'],
}
}

However, the command seems to require knowing the location of the file kernel.jl (corresponding seemingly to the install location of IJulia), and the above doesn't seem like it would fit with that.

I wanted to submit a PR along these lines, but I'm not sure how to deal with the issue of the kernel.jl file's location to make a PR that would actually work.

Read envkernel.py around KNOWN_KERNELS and compare its kernel.json handling with the IJulia argv shown in the issue. Investigate how the installed kernel.jl path can be resolved; done means an ijulia known-kernel entry can launch an IJulia installation without an unavailable hard-coded path.