Add function to get a repo's topics

[Hasnep]

Function to get a repo's topics. Fixes #195.

I wasn't sure where to put the function or what it should return. In the end I made it return a vector of Topic structs, but it could just return a vector of strings if that'd be better.

[maleadt]

I wasn't sure where to put the function or what it should return. In the end I made it return a vector of Topic structs, but it could just return a vector of strings if that'd be better.

I think that would be easier: topics aren't used by any other API, and are essentially just a String, so I propose we keep the interface simple?

I went ahead and did that change, while also adding PUT functionality to set topics.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 55.94%. Comparing base (809994e) to head (087c377).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
src/repositories/repositories.jl	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   56.22%   55.94%   -0.28%     
==========================================
  Files          36       36              
  Lines         843      849       +6     
==========================================
+ Hits          474      475       +1     
- Misses        369      374       +5     

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