Add escape_strings option to PrettyConfig

[juntyr]

This PR adds an escape_strings option to PrettyConfig that is enabled by default. When enabled, strings are serialised using escapes. When disabled, strings that contain no " are serialised literally. If the string does contain a ", a raw string literal with the minimum number of required leading # is emitted.

This feature was suggested in #425, i.e. this PR fixes #425.

• I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

Base: 87.22% // Head: 86.73% // Decreases project coverage by -0.48% ⚠️

Coverage data is based on head (5add5bd) compared to base (27d2de4).
Patch coverage: 81.44% of modified lines in pull request are covered.

❗ Current head 5add5bd differs from pull request most recent head 1a31674. Consider uploading reports for the commit 1a31674 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
- Coverage   87.22%   86.73%   -0.49%     
==========================================
  Files          58       59       +1     
  Lines        7145     7281     +136     
==========================================
+ Hits         6232     6315      +83     
- Misses        913      966      +53     

Impacted Files	Coverage Δ
src/ser/mod.rs	71.39% <64.70%> (-3.80%)	⬇️
tests/425_escape_strings.rs	100.00% <100.00%> (ø)
src/parse.rs	92.55% <0.00%> (+0.47%)	⬆️

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[juntyr]

These codecov results again make no sense. At some point I will need to investigate how to get them to be more stable, as on some weeks it works perfectly, and on some lines without code are marked as not covered :/

[juntyr]

Though I did notice that I introduced three uncovered test lines in #424 ...

[juntyr]

?r @torkleyy

[torkleyy]

❤️