split the config loader into file and toml parts

[Cadair]

I wanted to use the parser to parse the toml file I was loading into a little bot I am writing.

[codecov-io]

Codecov Report

Merging #118 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
+ Coverage   93.19%   93.22%   +0.02%     
==========================================
  Files          12       12              
  Lines         485      487       +2     
  Branches       53       53              
==========================================
+ Hits          452      454       +2     
  Misses         19       19              
  Partials       14       14

Impacted Files	Coverage Δ
src/towncrier/_settings.py	86.66% <100%> (+0.95%)	⬆️

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