add support for value mappings to stat panels.

[AndersSoee]

Fixes #222

Requires PR for sdk lib : K-Phoen/sdk#22

Enables usage in the form:

		stat.ValueMappings(
			stat.RangeMapping(0, 0.9).Text("DOWN").Color("red"),
			stat.RangeMapping(1, 1).Text("UP"),
			stat.ValueMapping("UP").Color("green"),
		),

[codecov]

Codecov Report

Merging #223 (15f8e87) into master (8a7600e) will increase coverage by 0.11%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   90.07%   90.18%   +0.11%     
==========================================
  Files          92       92              
  Lines        6934     7013      +79     
==========================================
+ Hits         6246     6325      +79     
  Misses        543      543              
  Partials      145      145              

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

[orloffv]

@K-Phoen, hi!
Grabana - great product, thank you.
Can you give feedback on this pull request, I need this feature