Add feature to convert any inline base64 images found to attachments

[jrbenriquez]

Inline Base64 images just does not work in Gmail anymore.

i.e.

<img align="center" border="0" src="data:image/png;base64,iVBORw0KGgoA.../>

I added this PR #89 just to see if there any interest from the maintainer to add this as a feature as this really helps me simplify my use of the library. Thanks! PR still lacks tests and I think can be further improved but just wanted to gauge interest first

It is enabled via the env var: EMAIL_CONVERT_BASE64_IMAGES which is False by default