Add optional string input to api's

[jaymarnz]

This adds an optional string input to each of the APIs so they can be chained without having to create temp files:

inlineScriptTags('./index.html') .then (htmlString => inlineStylesheets({ htmlPath: './index.html', htmlString })) .then (htmlString => inlineImages({ htmlPath: './index.html', htmlString })) .then (htmlString => fs.writeFileSync('./dist/index.html', htmlString));