Add support for using doc comments as descriptions

[LegNeato]

Fixes #194.

[codecov-io]

Codecov Report

Merging #195 into master will decrease coverage by 0.39%.
The diff coverage is 82.95%.

@@            Coverage Diff            @@
##           master     #195     +/-   ##
=========================================
- Coverage   89.88%   89.49%   -0.4%     
=========================================
  Files          96       96             
  Lines       17442    17827    +385     
=========================================
+ Hits        15678    15954    +276     
- Misses       1764     1873    +109

Impacted Files	Coverage Δ
juniper_tests/src/lib.rs	100% <ø> (ø)	⬆️
juniper_codegen/src/derive_input_object.rs	0% <0%> (ø)	⬆️
juniper_codegen/src/derive_enum.rs	0% <0%> (ø)	⬆️
juniper_codegen/src/derive_object.rs	0% <0%> (ø)	⬆️
juniper_tests/src/codegen/derive_enum.rs	100% <100%> (ø)	⬆️
juniper_codegen/src/util.rs	86.6% <82.89%> (+9.86%)	⬆️
juniper_tests/src/codegen/derive_input_object.rs	94.73% <90.32%> (-5.27%)	⬇️
juniper_tests/src/codegen/derive_object.rs	90.78% <92.4%> (+0.98%)	⬆️
... and 5 more

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[theduke]

@LegNeato I'll review later today

[LegNeato]

I'm going to be camping / offline all weekend so no rush!

[LegNeato]

Friendly poke @theduke