Use 'date' query parameter if available, instead of current date

[frontendtony]

This PR sets the serverless function to use the URL date query parameter where available and to default to the current date where unavailable.

How to test

Include a date query parameter like below and it will return tweets from that particular day from previous years, up to 2016

https://europe-west1-oldtweetstoday.cloudfunctions.net/getTweetsOnThisDay?username=oghie_js&from=2019&until=2016&date=2019-1-15

[shalvah]

This should be a day parameter, not a date.

Better yet, month and day parameters. That has several benefits:

• we won't need to worry about mm/dd or dd/mm
• anyone can parse it by looking at the URL
• we can still use today, then call setMonth() and setDay().

[frontendtony]

I have switched to using day and month parameters as advised and it does make more sense

Note that the month uses 1-based indexing (January === 1)

[shalvah]

Good work @Tonerolima. And yeah, 1-based index for the month is definitely the way to go.