Enforce field keys starting with a letter

[geoo89]

fixes #158

[fagiothree]

In RapidPro, the method that determines what is a valid key is
regex.match(r"^[a-z][a-z0-9_]*$", key, regex.V0)
are we also checking that the key only has letters or numbers or underscores?

[geoo89]

Thanks for the full specification. I updated the PR to match this.