Handle problems detected by used linters

The problems in the current code base detected by the linters that have been integrated in #62 through GolangCI should be handled by refactoring the affected implementations. This helps to improve the overall code quality and prevents possible errors.

Remove unused function parameters detected by unparam.

(*cmdOptions).prepare - cmd is unused: cmd/snowsaw/bootstrap/bootstrap.go:51:30

func (o *cmdOptions) prepare(cmd *cobra.Command, args []string) {
                           ^

(*cmdOptions).run - cmd is unused: cmd/snowsaw/bootstrap/bootstrap.go:100:26

func (o *cmdOptions) run(cmd *cobra.Command, args []string) {
                       ^

(*cmdOptions).run - args is unused: cmd/snowsaw/bootstrap/bootstrap.go:100:46

func (o *cmdOptions) run(cmd *cobra.Command, args []string) {
                                           ^

Improve function names and code flows detected by golint.
1. func NewJsonEncoder should be NewJSONEncoder: pkg/config/encoder/json/json.go:34:6
```
func NewJsonEncoder() Encoder {
   ^
```
2. var ExtensionsJson should be ExtensionsJSON: pkg/config/encoder/constants.go:26:2
```
ExtensionsJson = "json"
^
```
3. if block ends with a return statement, so drop this else and outdent its block (move short variable declaration to its own line if necessary): pkg/prt/printer.go:121:9
```
} else {
     ^
```
4. exported func Load returns unexported type *builder.builder, which can be annoying to use: pkg/config/builder/builder.go:39:32
```
func Load(files ...*file.File) *builder {
                             ^
```
Improve code style smells detected by gocritic.
1. assignOp: replace format = format + "\n" with format += "\n": pkg/prt/printer.go:179:4
```
format = format + "\n"
^
```
2. paramTypeCombine: func(v Verbosity, w io.Writer, prefix string, format string, args ...interface{}) could be replaced with func(v Verbosity, w io.Writer, prefix, format string, args ...interface{}): pkg/prt/printer.go:176:1
```
func (p *printerConfig) withNewLine(v Verbosity, w io.Writer, prefix string, format string, args ...interface{}) {
^
```
3. emptyStringTest: replace len(parts[0]) == 0 with parts[0] == "": pkg/snowblock/task/shell/shell.go:165:5
```
if len(parts[0]) == 0 {
 ^
```
4. elseif: can replace 'else {if cond {}}' with 'else if cond {}': cmd/snowsaw/bootstrap/bootstrap.go:57:9
```
} else {
     ^
```
Remove unnecessary type conversions detected by unconvert.
1. unnecessary conversion: pkg/prt/printer.go:132:16
```
*v = Verbosity(l)
            ^
```

arcticicestudio / snowsaw

Handle problems detected by used linters #80