Adds maxBuffer option that sets buffer limit for preprocess

[ilyavf]

Solves #20

This allows to avoid buffer overflow when running transformer with preprocess due to Node's default maxBuffer of 1Mb.
Docs + tests updated.

Default is 10Mb. Allows to override by providing maxBuffer option to transformer

Otherwise, if the source is large (e.g. using TailwindCSS) it can throw the following error:

> jest src

 FAIL  src/App.test.js
  ● Test suite failed to run

    spawnSync /bin/sh ENOBUFS

      at Object.process (node_modules/svelte-jester/src/transformer.js:15:17)
      at ScriptTransformer.transformSource (node_modules/@jest/transform/build/ScriptTransformer.js:464:35)
      at ScriptTransformer._transformAndBuildScript (node_modules/@jest/transform/build/ScriptTransformer.js:569:40)
      at ScriptTransformer.transform (node_modules/@jest/transform/build/ScriptTransformer.js:607:25)