Added internal packages for formatters, storage and models

[lonnblad]

Created a formatters pkg for the exported formatter symbols:

func FindFmt(name string) FormatterFunc
func Format(name, description string, f FormatterFunc)
func AvailableFormatters() map[string]string
type Formatter interface
type FormatterFunc func(string, io.Writer) Formatter

Created an internal pkg for the:

• all builtin formatters and the formatter-tests
• all internal models
• some shared internal utils

#319

[codecov]

Codecov Report

Merging #323 into master will decrease coverage by 4.54%.
The diff coverage is 95.47%.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
- Coverage   87.14%   82.59%   -4.55%     
==========================================
  Files          22       23       +1     
  Lines        2225     2218       -7     
==========================================
- Hits         1939     1832     -107     
- Misses        192      276      +84     
- Partials       94      110      +16     

Impacted Files	Coverage Δ
flags.go	93.91% <ø> (ø)
internal/formatters/undefined_snippets_gen.go	42.55% <ø> (ø)
internal/formatters/fmt.go	76.31% <76.31%> (ø)
internal/formatters/fmt_base.go	86.33% <84.61%> (ø)
internal/models/results.go	92.30% <92.30%> (ø)
suite.go	90.41% <97.43%> (ø)
internal/formatters/fmt_events.go	97.47% <98.11%> (ø)
internal/formatters/fmt_pretty.go	81.22% <98.14%> (ø)
fmt.go	100.00% <100.00%> (ø)
formatters/fmt.go	100.00% <100.00%> (ø)
... and 19 more

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