fix: Windows support

Fix an issue whereby the installer installed cheatsheets into the wrong
directory on Windows. This occurred because previously `path.Join` was
used where `path/filepath.Join` should have been used.

This matters, because the former always uses `/` as the path separator,
whereas the latter will use `/` or `\` as is appropriate for the
runtime environment.

This should resolve bullet point 4 in #665.

internal/sheet/copy.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"path"
+	"path/filepath"
 )
 
 // Copy copies a cheatsheet to a new location
@@ -22,7 +22,7 @@ func (s *Sheet) Copy(dest string) error {
 	defer infile.Close()
 
 	// create any necessary subdirectories
-	dirs := path.Dir(dest)
+	dirs := filepath.Dir(dest)
 	if dirs != "." {
 		if err := os.MkdirAll(dirs, 0755); err != nil {
 			return fmt.Errorf("failed to create directory: %s, %v", dirs, err)