fix(fields): escape Slack control characters in pullRequest title

[KENNYSOFT]

Related: #158

Actually I want to extract the escaping part but haven't find the best place to put that.

[codecov]

Codecov Report

Merging #178 (be41190) into master (5c8520d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #178   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          217       222    +5     
  Branches        56        60    +4     
=========================================
+ Hits           217       222    +5     

Impacted Files	Coverage Δ
src/fields.ts	100.00% <100.00%> (ø)

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