Added support for entries.

[Ardjan-Aalberts]

Created the entries resource. The entries can be created, updated and deleted. The archiving and publishing is also added into the resource, keep in mind that a resource can only be deleted when unpublished.

The content types now use the name as it's ID. This makes the Terraform configuration a bit easier. The entry resource does the same, by setting the entry_id property as ID in Contentful. This creates a PUT request which will create an entry with the specified id.

[codecov]

Codecov Report

Merging #5 into master will increase coverage by 0.24%.
The diff coverage is 68.20%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   67.13%   67.37%   +0.24%     
==========================================
  Files           6        8       +2     
  Lines        1004     1220     +216     
==========================================
+ Hits          674      822     +148     
- Misses        269      316      +47     
- Partials       61       82      +21     

Impacted Files	Coverage Δ
contentful/resource_contentful_contenttype.go	65.75% <ø> (ø)
contentful/provider.go	93.75% <66.66%> (+0.41%)	⬆️
contentful/resource_contentful_entry.go	67.13% <67.13%> (ø)
contentful/resource_contentful_environment.go	70.42% <70.42%> (ø)

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