Handle TOML Library Licensing

This patch updates the package used for parsing TOML content from
"github.com/BurntSushi/toml" to "github.com/pelletier/go-toml" as the
latter uses a more accepted OSS license (MIT), enabling the inclusion of
Viper or projects that depend on Viper in projects that have licensing
requirements incongruent with the license of the previous TOML package.

Closes #228
Closes #225
Fixes #179

util.go

diff --git a/util.go b/util.go
index 46f923c..fe6cb45 100644
--- a/util.go
+++ b/util.go
@@ -21,9 +21,9 @@
 	"strings"
 	"unicode"
 
-	"github.com/BurntSushi/toml"
 	"github.com/hashicorp/hcl"
 	"github.com/magiconair/properties"
+	toml "github.com/pelletier/go-toml"
 	"github.com/spf13/cast"
 	jww "github.com/spf13/jwalterweatherman"
 	"gopkg.in/yaml.v2"
@@ -155,9 +155,14 @@
 		}
 
 	case "toml":
-		if _, err := toml.Decode(buf.String(), &c); err != nil {
+		tree, err := toml.LoadReader(buf)
+		if err != nil {
 			return ConfigParseError{err}
 		}
+		tmap := tree.ToMap()
+		for k, v := range tmap {
+			c[k] = v
+		}
 
 	case "properties", "props", "prop":
 		var p *properties.Properties