cmd: initialize config fields before use

[badouralix]

Fixes #99 by introducing config constructors
Note that this is one way to fix the issue, alternatives can be considered too

Without any .yamlfmt, yamlfmt behaves as expected, here below with debug logs :

$ rm .yamlfmt

$ go run ./cmd/yamlfmt
{Extensions:[yaml yml] Include:[] Exclude:[] Doublestar:false LineEnding:lf FormatterConfig:0x14000120168}
{Type: FormatterSettings:map[]}

$ go run ./cmd/yamlfmt --formatter retain_line_breaks=true
{Extensions:[yaml yml] Include:[] Exclude:[] Doublestar:false LineEnding:lf FormatterConfig:0x14000118150}
{Type: FormatterSettings:map[retain_line_breaks:true]}

Same expected behavior when .yamlfmt contains no overrides :

$ cat .yamlfmt
formatter:
  type: basic

$ go run ./cmd/yamlfmt
{Extensions:[yaml yml] Include:[] Exclude:[] Doublestar:false LineEnding:lf FormatterConfig:0x14000124168}
{Type:basic FormatterSettings:map[]}

$ go run ./cmd/yamlfmt --formatter retain_line_breaks=true
{Extensions:[yaml yml] Include:[] Exclude:[] Doublestar:false LineEnding:lf FormatterConfig:0x1400000c180}
{Type:basic FormatterSettings:map[retain_line_breaks:true]}

Same expected behavior when .yamlfmt contains at least one overrides :

$ cat .yamlfmt
formatter:
  type: basic
  retain_line_breaks: false

$ go run ./cmd/yamlfmt
{Extensions:[yaml yml] Include:[] Exclude:[] Doublestar:false LineEnding:lf FormatterConfig:0x14000114168}
{Type:basic FormatterSettings:map[retain_line_breaks:false]}

$ go run ./cmd/yamlfmt --formatter retain_line_breaks=true
{Extensions:[yaml yml] Include:[] Exclude:[] Doublestar:false LineEnding:lf FormatterConfig:0x140000a0168}
{Type:basic FormatterSettings:map[retain_line_breaks:true]}

[braydonk]

For some reason I thought mapstructure would just decode to an empty map instead of nil so I neglected to try this use case. Thank you very much for the fix!