Support `npx` command

[terasakisatoshi]

This PR allows us to call npx command from NodeJS.jl

After this PR is merged, we can use it as follows:

using NodeJS

open("sample.md", "w") do io
	write(io,"# Hello NodeJS\n")
end

mdfile = joinpath(@__DIR__, "sample.md")
# https://github.com/marp-team/marp-cli#try-it-now
# sample.html will be generated
run(`$(npx_cmd()) @marp-team/marp-cli $(mdfile)`)