Vendor all dependencies (Warning: Huge changeset.)

The vendor/ folder was created with the help of @FiloSottile's gvt and
vendorcheck.

Any dependencies of Caddy plugins outside this repo are not vendored.

We do not remove any unused, vendored packages because vendorcheck -u
only checks using the current build configuration; i.e. packages that
may be imported by files toggled by build tags of other systems.

CI tests have been updated to ignore the vendor/ folder. When Go 1.9 is
released, a few of the go commands should be revised to again use ./...
as it will ignore the vendor folder by default.

vendor/github.com/alecthomas/units/si.go

@@ -0,0 +1,26 @@
+package units
+
+// SI units.
+type SI int64
+
+// SI unit multiples.
+const (
+	Kilo SI = 1000
+	Mega    = Kilo * 1000
+	Giga    = Mega * 1000
+	Tera    = Giga * 1000
+	Peta    = Tera * 1000
+	Exa     = Peta * 1000
+)
+
+func MakeUnitMap(suffix, shortSuffix string, scale int64) map[string]float64 {
+	return map[string]float64{
+		shortSuffix:  1,
+		"K" + suffix: float64(scale),
+		"M" + suffix: float64(scale * scale),
+		"G" + suffix: float64(scale * scale * scale),
+		"T" + suffix: float64(scale * scale * scale * scale),
+		"P" + suffix: float64(scale * scale * scale * scale * scale),
+		"E" + suffix: float64(scale * scale * scale * scale * scale * scale),
+	}
+}