make csrfName configurable; aware of response filePath

[mechiland]

Rails need some special csrfToken name when uploading files. This PR will add another option to config image upload.

Usage:

imageCSRFName: 'authenticity_token',
imageCSRFToken: document.querySelector("meta[name=csrf-token]").getAttribute("content"),

Also, this PR contains a patch to be aware of returned filePath is an absolute path (won't add window.location.origin in front of file)