Add the @has_preference and @delete_preferences convenience functions and macros

[DilumAluthge]

Fixes #4

This pull request adds the following convenience functions, with associated convenience macros:

1. @has_preference
2. @delete_preferences

@has_preference is pretty straightforward - if the result of @load_preference is nothing, it returns false, otherwise it returns true.

For @delete_preferences, there is a keyword argument block_inheritance. If block_inheritance is false (which is the default), then we call @set_preferences with missing as the value for each given preference key. If block_inheritance is true, then we call @set_preferences with nothing as the value for each given preference key.

[staticfloat]

Overall, this looks perfect.

[DilumAluthge]

BTW, it would be best to first merge #7, and then I can rebase this PR. That way we can double check that CI passes on this PR.

EDIT: this is done now.

[codecov]

Codecov Report

Merging #6 (bae9101) into master (e517d28) will decrease coverage by 5.11%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
- Coverage   81.81%   76.69%   -5.12%     
==========================================
  Files           2        2              
  Lines          88      103      +15     
==========================================
+ Hits           72       79       +7     
- Misses         16       24       +8     

Impacted Files	Coverage Δ
src/Preferences.jl	76.40% <46.66%> (-6.03%)	⬇️

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[DilumAluthge]

Alright, should be ready for another round of review.

[timholy]

Curious about this.

[timholy]

Hmm, GitHub seems to be having some troubles. Issue is that has_preference takes a default argument but doesn't use it. Typo, or intended?

[DilumAluthge]

Oh that's definitely a bug.

[DilumAluthge]

It shouldn't take a default argument. The docs in https://github.com/JuliaPackaging/Preferences.jl/blob/master/README.md#api are correct, but the function itself is incorrect.

[timholy]

xref #29