allow using environment vars for configuration

fixes #151

third_party/envy/LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2017 Jamie Alquiza
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

third_party/envy/README.md

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+envy is a copy of https://github.com/jamiealquiza/envy without the cobra
+support.

third_party/envy/envy.go

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+// Package envy automatically exposes environment
+// variables for all of your flags.
+package envy
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// Parse takes a prefix string and exposes environment variables
+// for all flags in the default FlagSet (flag.CommandLine) in the
+// form of PREFIX_FLAGNAME.
+func Parse(p string) {
+	update(p, flag.CommandLine)
+}
+
+// update takes a prefix string p and *flag.FlagSet. Each flag
+// in the FlagSet is exposed as an upper case environment variable
+// prefixed with p. Any flag that was not explicitly set by a user
+// is updated to the environment variable, if set.
+func update(p string, fs *flag.FlagSet) {
+	// Build a map of explicitly set flags.
+	set := map[string]interface{}{}
+	fs.Visit(func(f *flag.Flag) {
+		set[f.Name] = nil
+	})
+
+	fs.VisitAll(func(f *flag.Flag) {
+		// Create an env var name
+		// based on the supplied prefix.
+		envVar := fmt.Sprintf("%s_%s", p, strings.ToUpper(f.Name))
+		envVar = strings.Replace(envVar, "-", "_", -1)
+
+		// Update the Flag.Value if the
+		// env var is non "".
+		if val := os.Getenv(envVar); val != "" {
+			// Update the value if it hasn't
+			// already been set.
+			if _, defined := set[f.Name]; !defined {
+				fs.Set(f.Name, val)
+			}
+		}
+
+		// Append the env var to the
+		// Flag.Usage field.
+		f.Usage = fmt.Sprintf("%s [%s]", f.Usage, envVar)
+	})
+}

third_party/envy/go.mod

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+module .
+
+go 1.12