Fix inlining when using a name other than glslify

Previously, glslify was assuming the variable name used for requiring would be "glslify". This would mean the following would not work:

``` javascript
var createShader = require('glslify')

createShader({
    vertex: './file.vert'
   , fragment: './file.frag'
})
```

And the following would still get replaced:

``` javascript
glslify('no requires')
```

Using the [sleuth](http://github.com/hughsk/sleuth) module makes it easy to discover what variable name is being used and use it with replace-method.

Have also included preemptive checks to bail before parsing files which definitely don't contain calls to "glslify" within.

@chrisdickinson let me know if you have any thoughts on this before I merge, if you have the time :)