Map camelCased keys to fields in structs (#251)

The name for each field in a struct is used to look up a key in the TOML
tree.  A few different (case-sensitive) forms of this name are tried.
Previously, the current, lower-cased, and title-cased versions of the
name are tried.  This precludes camelCased keys from mapping back to
fields in structs.  This change adds camelCase to the set of keys to
try.

For example, the following TOML:

  fooBar = 10

Would previously *not* map to the following struct:

  type Foo struct {
    FooBar int
  }

This change corrects this.

marshal.go

@@ -470,7 +470,12 @@ func (d *Decoder) valueFromTree(mtype reflect.Type, tval *Tree) (reflect.Value,
 			opts := tomlOptions(mtypef, an)
 			if opts.include {
 				baseKey := opts.name
-				keysToTry := []string{baseKey, strings.ToLower(baseKey), strings.ToTitle(baseKey)}
+				keysToTry := []string{
+					baseKey,
+					strings.ToLower(baseKey),
+					strings.ToTitle(baseKey),
+					strings.ToLower(string(baseKey[0])) + baseKey[1:],
+				}
 				for _, key := range keysToTry {
 					exists := tval.Has(key)
 					if !exists {