Parallel processing with one PhantomJS process?

Thanks for this wonderful module!

I use a promise-based approach to convert multiple markdowns in parallel, like so:

```js

const markdownpdf = require( 'markdown-pdf' )
const pdf = markdownpdf( )
const fs = require( 'fs' )

const mdtopdf = ( path, md ) => {
return new Promise( ( resolve, reject ) => {
// Check if pdf is already there
if( fs.existsSync( path ) ) resolve( 'File already exists' )
// If not, generate it
pdf.from.string( md, path ) )
.to( output, f => {
resolve( output )
} )
} )
}

const publishpdf = ( path, markdowns ) => {
return Promise.all( markdowns.map( md => mdtopdf( path, md ) ) )
}
```

This means that when I run my build there are a **lot** of PhantonJs processes that make my laptop go wonky (MBP just starts the fan, small MB buckles and freezes for a bit).

Is there a way to to parallel building using a single PhantomJs process? Perhaps by starting a Phantom instance and having the module hook into an already running instance?

edit: removed some useless context code