Adds additional query syntax for the CDA

[floriank]

After having no time for a while these days and thinking a bit about #38, I decided to put in some work about it.

This will add support for more complex queries as noted by @ryansch, in the form of:

Entries 
|> content_type("my_content_type") 
|> by(id: "my_id", number_of_warthogs: [lt: 42]) # or similar, no clue what offers the most benefits
|> fetch_all

and provide some safeguards around it, e.g. using by without a content_type call is not permitted, etc.

Feedback, as always, is very welcome.

TODO

• [x] Entries by content_type
• [x] Entries and Assets via by
• [x] Array support in by, via [nin] and [in]
• [x] Range support in by via [lte], [gte], [lt] and [gt]
• [x] Existence support
• [x] Full text support via query and [match]

Out of scope

• Full text per field
• Geolocation shenanigans
• filtering by mime types

Somebody create some issues for the ones above please :wink: (will probably do tomorrow)

Fixes #38.

[coveralls]

Pull Request Test Coverage Report for Build 1822167b055a772b4c6d481759eee4d9baa79915-PR-49

• 41 of 48 (85.42%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+11.6%) to 79.412%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/contentful_delivery/delivery.ex	2	3	66.67%
lib/contentful/query.ex	19	25	76.0%
<!--	Total:	41	48	85.42%	-->

Totals
Change from base Build 50ea35dcac6ffacfcfa51f7ce5d8b4e222b16d5c:	11.6%
Covered Lines:	108
Relevant Lines:	136

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💛 - Coveralls