Support $GO111MODULE with Go tooling

Allows the user to override $GO111MODULE environment variable through
ale options. This gives control over the default behavior of Go module
resolution.

Golang documentation:
https://github.com/golang/go/wiki/Modules#how-to-use-modules

Add `ale#Go#EnvString()` function to make it easy to add similar Go
environment variables in the future.

Use the new `EnvString` function in all available Go tools callbacks
& update tests

Also add test of linter command callback for `gofmt`

ale_linters/go/staticcheck.vim

@@ -8,17 +8,18 @@ function! ale_linters#go#staticcheck#GetCommand(buffer) abort
     let l:filename = expand('#' . a:buffer . ':t')
     let l:options = ale#Var(a:buffer, 'go_staticcheck_options')
     let l:lint_package = ale#Var(a:buffer, 'go_staticcheck_lint_package')
+    let l:env = ale#go#EnvString(a:buffer)
 
     " BufferCdString is used so that we can be sure the paths output from
     " staticcheck can be calculated to absolute paths in the Handler
     if l:lint_package
         return ale#path#BufferCdString(a:buffer)
-        \   . 'staticcheck'
+        \   . l:env . 'staticcheck'
         \   . (!empty(l:options) ? ' ' . l:options : '') . ' .'
     endif
 
     return ale#path#BufferCdString(a:buffer)
-    \   . 'staticcheck'
+    \   . l:env . 'staticcheck'
     \   . (!empty(l:options) ? ' ' . l:options : '')
     \   . ' ' . ale#Escape(l:filename)
 endfunction