Support inline attachments

[axelson]

Inline attachment support was added in Bamboo 1.4 so I've also updated the minimum required version of bamboo to 1.4.0. As part of this the default json library in bamboo changed to Jason so we need to set configuration to keep using Poison (but this configuration is only used for this library itself so it doesn't impact users)

Note: When I ran mix deps.get, there were a significant updates:

jason@Jasons-MacBook-Pro ~/d/f/bamboo_postmark (attachment-cid)> mix deps.get
Resolving Hex dependencies...
Dependency resolution completed:
Unchanged:
  cowboy 1.1.2
  cowlib 1.0.2
  earmark 1.3.1
  ex_doc 0.19.3
  makeup 0.8.0
  makeup_elixir 0.13.0
  metrics 1.0.1
  nimble_parsec 0.5.0
  parse_trans 3.3.0
  plug_cowboy 1.0.0
  ranch 1.3.2
Upgraded:
  bamboo 1.2.0 => 1.5.0
  certifi 2.4.2 => 2.5.2
  hackney 1.15.0 => 1.16.0
  idna 6.0.0 => 6.0.1
  mime 1.3.1 => 1.4.0
  mimerl 1.0.2 => 1.2.0
  plug 1.7.2 => 1.10.4
  plug_crypto 1.0.0 => 1.2.0
  poison 3.0.0 => 4.0.1 (major)
  ssl_verify_fun 1.1.4 => 1.1.6
  unicode_util_compat 0.4.1 => 0.5.0 (minor)
New:
  telemetry 0.4.2
All dependencies are up to date

These changes also don't impact users because only the contents of the
mix.exs file matter to users of this library.

[pablo-co]

Thank you for your contribution @axelson. This looks good to me. Merging this in and releasing as 0.7.0.

[axelson]

Thanks for merging and thanks for maintaining bamboo_postmark!