Some ergonomics improvements

[Lucas-C]

Hi.

This PR introduces a few things:

• interactive form to create you own grid without having to edit the JS code
• control over the max grid growth and whether to allow for blank cells or not
• the ability to specify a secret word whose letters occupy the remaining squares
• a new newPuzzleLax function in wordfind.js that can, as a fallback, try to build a grid without some words
• the HTML + CSS has been modified to allow a responsive rendering on mobile phones

[Lucas-C]

Thanks for the merge !

Sadly, I think I have pushed extra commits on this branch after sending this PR. My mistake, sorry.

As a consequence, I almost emptied the README. You may want to restore your old one.

I also added my name in the index.html: feel free to remove it.