• open root directory in console and type 'node index.js' or 'npm start'
• allow user to supply a CLI argument containing a comma-separated list
  • if no argument is given, load tags.txt
• count how many times each list item appears within the objects in data/*.json (note: objects can be nested).
• final output should be formatted like this (sorted by most popular tag first):

pizza     15
spoon     2
umbrella  0
cats      0

• cache the result so that subsequent runs can return the result immediately without needing to process the files.
• use only core modules
• use asynchronous variants of the file IO functions (eg. use fs.readFile not fs.readFileSync).
• if any of the data files contain invalid JSON, log the error with console.error and continue, ignoring that file.
• use latest version of node (at time of writing), but use plain callbacks instead of promises.