Added 'compatible-with' (caret ^) operator

[cmaglie]

I've added the "compatible-with" operation, I'd like to make this operation the default instead of the "equals" as in #7.
When this is merged I'll redo #7.

I've also changed the table in the readme because:

• a constraint is basically a boolean condition, it may match or not match against a version: 1.0.0 is >=0.5.0? -> true
• a constraint may reduce the set of releases on which a dependency resolver operates: so given the same set of releases the resolver may output a different solution if different constraints are given

[codecov]

Codecov Report

Merging #8 (197ec76) into master (371234f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #8   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          735       761   +26     
=========================================
+ Hits           735       761   +26     

Impacted Files	Coverage Δ
constraints.go	100.00% <100.00%> (ø)
relaxed_version.go	100.00% <100.00%> (ø)
version.go	100.00% <100.00%> (ø)

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