Implement configurable tag delimiter; default @

[bndnsmth]

This implements an option to configure the character(s) used as the tag delimiter with a default of @ for backwards compatibility. In PowerShell, @ is used as the array operator. As a result, any input pre-fixed with @ is not passed to the program unless escaped in a string. This will allow users to specify their own prefix.

Demo of issue:

$ cat .\argv.js
console.log(process.argv)

$ node .\argv.js -t @board Sample task
[ 'C:\\node.exe',
  'C:\\argv.js',
  '-t',
  'Sample',
  'task' ]

---

Related: #148, #41