Check for startup_ijulia.jl and load it if present (#745)

Author: timholy

README.md

@@ -170,6 +170,10 @@ running IJulia kernel.
 If your code needs to detect whether it is running in an IJulia notebook
 (or other Jupyter client), it can check `isdefined(Main, :IJulia) && Main.IJulia.inited`.
 
+### Customizing your IJulia environment
+
+If you want to run code every time you start IJulia---but only when in IJulia---add a `startup_ijulia.jl` file to your Julia `config` directory, e.g., `~/.julia/config/startup_ijulia.jl`.
+
 ### Julia and IPython Magics
 
 One difference from IPython is that the IJulia kernel does

src/kernel.jl

@@ -8,6 +8,9 @@ end
 
 IJulia.init(ARGS)
 
+startupfile = abspath(homedir(), ".julia", "config", "startup_ijulia.jl")
+isfile(startupfile) && Base.include(Main, startupfile)
+
 # import things that we want visible in IJulia but not in REPL's using IJulia
 import IJulia: ans, In, Out, clear_history