Parameterization of options for jupyter kernelspec install in installkernel()

[setempler]

Hi,

I had the problem to not be able to apply IJulia.installspec correctly for a configuration,
where a jupyter notebook server was installed in a python virtual environment (venv).

With python, it is possible to register a kernel installed in a venv into a server
that is also installed in a (different) venv. Key is to use the --prefix argument, such as in:

### bash

# install server in a python venv
export tmpd=$HOME/tmp/ijulia
mkdir -p $tmpd
python3 -m venv $tmpd # 3.7.0
source $tmpd/bin/activate
pip install jupyter
deactivate

# install a python kernel venv
python3 -m venv $tmpd/kernelenv
source $tmpd/kernelenv/bin/activate
pip install ipykernel
python -m ipykernel install \
    --name virtual_python \
    --prefix="$tmpd" \
    --display-name "Virtual Python"
deactivate

# verify
source $tmpd/bin/activate
jupyter kernelspec list
deactivate

So I request a pull for a change to allow customisation of the arguments
forwarded to jupyter kernelspec install to allow registering a Julia kernel
from IJulia.installspec to a server installed in a venv.

### julia

# install
envd = ENV["HOME"] * "/tmp/ijulia"
using IJulia
IJulia.installkernel("Virtual Julia", jupyter_options = ["--prefix='$envd'"])

### bash

# verify
source $tmpd/bin/activate
jupyter kernelspec list
deactivate

System:

• macOS 10.13.6
• bash 4.4.23
• python 3.7.0

[setempler]

I found

LoadError: function cat does not accept keyword arguments
while loading /home/travis/.julia/v0.6/IJulia/deps/build.jl, in expression starting on line 125

at travis - is this (compatibility to 0.6) an issue?

[setempler]

Closing due to extended version in #719