Adds support for sendgrid-python v6

Now supports the newest version of sendgrid's api (https://github.com/sendgrid/sendgrid-python/releases/tag/v6.0.0). Maintains backwards compatibility.

Adds output to console option

Thanks @liavkoren for your contribution!

#45

Adds Dynamic Data Option

Thanks @blackfire70 for #39

Bump to 0.7.0 to clean up semantic versioning

Bug Fixes

1. Adds ASM and suppression group support
2. Strips brackets from content-id since they are added back by sendgrid's api

Adds Sandbox Mode in DEBUG

Now supports Sendgrid's sandbox mode in DEBUG mode (by default).

https://sendgrid.com/docs/Classroom/Send/v3_Mail_Send/sandbox_mode.html

Thanks to @Fandekasp for the code review and feature request

Bug Fix: Adds text/plain content to EmailMultiAlternatives

Fix per issue #2

Removes alpha tag

v0.6.6

Removes alpha tag

Updates sendgrid dependencies

Due to breaking changes in sendgrid-python's newest v5.0.0 release: https://github.com/sendgrid/sendgrid-python/releases/tag/v5.0.0

Pins python-sendgrid

Per this discussion: sendgrid/sendgrid-python#328 (comment)

Will unpin once python-sendgrid unpins its dependency to python-http-client (most likely in the v5.0.0 release)

v0.6.0a

Increment version