zap: Add ParseAtomicLevel func

[Techassi]

As discussed in #1047 this PR adds a zap.ParseAtomicLevel() func which enables the user to construct a log level based on ASCII text input.

Usage

level, err := zap.ParseAtomicLevel("info") // InfoLevel, nil

level, err := zap.ParseAtomicLevel("DEBUG") // DebugLevel, nil

level, err := zap.ParseAtomicLevel("FOO") // InfoLevel, "unrecognized level: "FOO""

[codecov]

Codecov Report

Merging #1048 (ed6314c) into master (e751939) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1048   +/-   ##
=======================================
  Coverage   98.11%   98.12%           
=======================================
  Files          48       48           
  Lines        2074     2080    +6     
=======================================
+ Hits         2035     2041    +6     
  Misses         30       30           
  Partials        9        9           

Impacted Files	Coverage Δ
level.go	100.00% <100.00%> (ø)

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[sywhang]

Thank you for the contributions! 👍