Engine registration and custom options

[dcwatson]

This was originally part of #151. The short version is that this adds a register function to allow third-party database engines to be registered as URL schemes. It also lets you pass any DATABASES setting options through config/parse (see #116 for example).

[codecov]

Codecov Report

Merging #170 (fcffaf1) into master (389febf) will increase coverage by 5.34%.
The diff coverage is 89.13%.

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   79.72%   85.07%   +5.34%     
==========================================
  Files           1        1              
  Lines          74       67       -7     
  Branches       14       19       +5     
==========================================
- Hits           59       57       -2     
+ Misses          9        6       -3     
+ Partials        6        4       -2     

Impacted Files	Coverage Δ
dj_database_url.py	85.07% <89.13%> (+5.34%)	⬆️

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[dcwatson]

This is a breaking change in terms of the backends registered by default. I'm only registering the backends in Django itself, so no Redshift, Cockroach, MSSQL, etc. I could be convinced otherwise, but my thought was we probably want to get out of the business of blessing third-party database engines and just make it as easy as possible for people to use whatever they like.

I should mention, my original intention was to fork the project, so I hadn't written any documentation for this yet. I'm (obviously) inclined to merge this, but would love a second set of eyes on it. I'll also work on corresponding updates to the README to document the registration process, and the breaking change above.

[mattseymour]

@dcwatson thanks for the submission, we have had two very similar yet competing PRs come in within a couple of hours of each other. Can I direct you to #171 and see if the changes you made can work along side this PR.

📓 I have put a similar message on #171

[dcwatson]

I'll close this in favor of #170. I still think dropping pre-2.0 Django support and not automatically registering third-party engines are worthy goals, but can be separate issues.