Allow arrays in markdown template literal

[jstcki]

Concatenate arrays to output in markdown template literals instead of JSON-stringifying them. A better version would also handle the content of the array but this is fine as a small improvement.

Partially fixes #403

[jstcki]

@Cethy, this allows you to do something like this:

markdown`
# Hello

${["foo", "bar"].map(x => [
  `## More markdown`,
  <div>{x}</div>
])}
`