Make fzf pass go vet

Add String() methods to types, so they can be printed with %s. Change
some %s format specifiers to %v, when the default string representation
is good enough. In Go 1.10, `go test` triggers a parallel `go vet`. So
this also makes fzf pass `go test`.

Close #1236
Close #1219

src/pattern.go

@@ -1,6 +1,7 @@
 package fzf
 
 import (
+	"fmt"
 	"regexp"
 	"strings"
 
@@ -34,6 +35,11 @@ type term struct {
 	caseSensitive bool
 }
 
+// String returns the string representation of a term.
+func (t term) String() string {
+	return fmt.Sprintf("term{typ: %d, inv: %v, text: []rune(%q), caseSensitive: %v}", t.typ, t.inv, string(t.text), t.caseSensitive)
+}
+
 type termSet []term
 
 // Pattern represents search pattern