Fix linter warnings

[timfallmk]

I've submitted fixes for the critical linter failures in #69 and #68, but there remains a large number of non-critical warnings. In an effort to clean up the code a little, we should probably take care of these.

I'm using gometalinter to run a number of standard linters at once. My command, along with the output is below.

go get github.com/alecthomas/gometalinter
gometalinter --install


gometalinter --vendor --skip testdata -t --aggregate ./...
cli.go:76::warning: cyclomatic complexity 21 of function (*CLI).Run() is high (> 10) (gocyclo)
github_test.go:180::warning: declaration of "err" shadows declaration at github_test.go:167 (vetshadow)
github_test.go:194::warning: declaration of "err" shadows declaration at github_test.go:167 (vetshadow)
cli.go:34:1:warning: ExitCodeRepoNotFound is unused (deadcode)
cli.go:51:2:warning: exported const DefaultParallel should have comment (or a comment on this block) or be unexported (golint)
cli.go:20:1:warning: EnvStackTrace is unused (deadcode)
ghr_test.go:195::warning: declaration of "err" shadows declaration at ghr_test.go:164 (vetshadow)
github_test.go:173::warning: declaration of "err" shadows declaration at github_test.go:167 (vetshadow)
github_test.go:179::warning: declaration of "filename" shadows declaration at github_test.go:178 (vetshadow)
cli.go:34:1:warning: ExitCodeInvalidURL is unused (deadcode)
cli.go:22::warning: Potential hardcoded credentials,HIGH,LOW (gas)
cli.go:270::warning: declaration of "err" shadows declaration at cli.go:232 (vetshadow)
cli.go:279::warning: declaration of "err" shadows declaration at cli.go:232 (vetshadow)
cli_test.go:37::warning: declaration of "err" shadows declaration at cli_test.go:28 (vetshadow)
ghr_test.go:170::warning: declaration of "err" shadows declaration at ghr_test.go:164 (vetshadow)
ghr_test.go:180::warning: declaration of "err" shadows declaration at ghr_test.go:164 (vetshadow)
cli_test.go:33::warning: declaration of "err" shadows declaration at cli_test.go:28 (vetshadow)
ghr_test.go:37::warning: cyclomatic complexity 12 of function TestGHR_CreateReleaseWithExistingRelease() is high (> 10) (gocyclo)
cli.go:30:2:warning: exported const EnvStackTrace should have comment (or a comment on this block) or be unexported (golint)
cli.go:61:1:warning: comment on exported function PrintRedf should be of the form "PrintRedf ..." (golint)
cli.go:34:1:warning: ExitCodeRleaseError is unused (deadcode)
ghr.go:27:5:warning: error return value not checked (non-boolean condition in if statement) (errcheck)
ghr.go:27:5:warning: non-boolean condition in if statement (gotype, interfacer, maligned, megacheck, unconvert)
ghr.go:27:5:warning: unused struct field non-boolean condition in if statement (structcheck)
github_test.go:98::warning: cyclomatic complexity 11 of function TestGitHubClient_Upload() is high (> 10) (gocyclo)
github_test.go:112::warning: declaration of "err" shadows declaration at github_test.go:106 (vetshadow)
ghr.go:27:5:warning: unused variable or constant non-boolean condition in if statement (varcheck)

[Songmu]

Thank you for reporting.
I confirmed them, but there was no critical problem.
I think that using golint and govet are sufficient in CI, so close this issue.