Support image alt text in Airtable plugin

[madebyisaacr]

The Airtable plugin supports importing images but there's no way to add alt text to them.

We can reuse the format from CSV Import and Export plugins, which is to add another column with the :alt suffix.

For example, you add two columns in Airtable: an attachment property and a text property with the same name plus an :alt suffix. The plugin would automatically link them and add it as alt text on the image.