main: switch to urfave/cli/v2

For now this is a drop in replacement for all the flags curently
implemented

Fixes: #171

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

vendor/github.com/urfave/cli/v2/args.go

@@ -0,0 +1,54 @@
+package cli
+
+type Args interface {
+	// Get returns the nth argument, or else a blank string
+	Get(n int) string
+	// First returns the first argument, or else a blank string
+	First() string
+	// Tail returns the rest of the arguments (not the first one)
+	// or else an empty string slice
+	Tail() []string
+	// Len returns the length of the wrapped slice
+	Len() int
+	// Present checks if there are any arguments present
+	Present() bool
+	// Slice returns a copy of the internal slice
+	Slice() []string
+}
+
+type args []string
+
+func (a *args) Get(n int) string {
+	if len(*a) > n {
+		return (*a)[n]
+	}
+	return ""
+}
+
+func (a *args) First() string {
+	return a.Get(0)
+}
+
+func (a *args) Tail() []string {
+	if a.Len() >= 2 {
+		tail := []string((*a)[1:])
+		ret := make([]string, len(tail))
+		copy(ret, tail)
+		return ret
+	}
+	return []string{}
+}
+
+func (a *args) Len() int {
+	return len(*a)
+}
+
+func (a *args) Present() bool {
+	return a.Len() != 0
+}
+
+func (a *args) Slice() []string {
+	ret := make([]string, len(*a))
+	copy(ret, *a)
+	return ret
+}