added tests that run in headless browser

high level execute() function that:

• supports array commands syntax ['convert', 'foo.png','bar.png'] and CLI like commands i.e: 'convert foo.png bar.png'
• supports running several commands serially reusing output images as input images in following commands (see spec/executeSpec.ts)

./src/util/file.ts contains utilities to easy MagickInputFile creation and transformation between input files and output files, blobs, arrays, strings, etc

./src/util/image.ts contains utilities related to images. Right now has compare() and extractInfo() that help me in the tests