add a way to specify the X11 display in Linux

[illegalprime]

When running a process using this as a service I get a segfault from node.js like so:

[michael] [test_robotjs] $ node index.js      
Could not open main display
zsh: segmentation fault (core dumped)  node index.js

index.js is just trying to do anything with robot.js:

var robotjs = require("robotjs");
robotjs.mouseClick();

The workaround is easy (run with DISPLAY=:0 or similar), but I would rather not have to configure this module through environment variables.

[octalmage]

See #34 and let me know if that works for you!

[illegalprime]

Will test it out later today, sorry for the delay.

[octalmage]

No worries! You're doing me a favor!