Fixes #6: implement \uXXXX escaping

lexer.go

diff --git a/lexer.go b/lexer.go
index ba18a9b..09598ec 100644
--- a/lexer.go
+++ b/lexer.go
@@ -6,6 +6,7 @@
 import (
 	"fmt"
 	"regexp"
+	"strconv"
 	"strings"
 	"unicode"
 	"unicode/utf8"
@@ -70,6 +71,11 @@
 	return unicode.IsNumber(r)
 }
 
+func isHexDigit(r rune) bool {
+	return isDigit(r) ||
+		r == 'A' || r == 'B' || r == 'C' || r == 'D' || r == 'E' || r == 'F'
+}
+
 // Define lexer
 type lexer struct {
 	input  string
@@ -323,6 +329,23 @@
 		} else if l.follow("\\\\") {
 			l.pos += 1
 			growing_string += "\\"
+		} else if l.follow("\\u") {
+			l.pos += 2
+			code := ""
+			for i := 0; i < 4; i++ {
+				c := l.peek()
+				l.pos += 1
+				if !isHexDigit(c) {
+					return l.errorf("unfinished unicode escape")
+				}
+				code = code + string(c)
+			}
+			l.pos -= 1
+			intcode, err := strconv.ParseInt(code, 16, 32)
+			if err != nil {
+				return l.errorf("invalid unicode escape: \\u" + code)
+			}
+			growing_string += string(rune(intcode))
 		} else {
 			growing_string += string(l.peek())
 		}

lexer_test.go

diff --git a/lexer_test.go b/lexer_test.go
index de0ccc6..6723738 100644
--- a/lexer_test.go
+++ b/lexer_test.go
@@ -300,3 +300,12 @@
 		token{tokenEOF, ""},
 	})
 }
+
+func TestKeyEqualStringUnicodeEscape(t *testing.T) {
+	testFlow(t, "foo = \"hello \\u2665\"", []token{
+		token{tokenKey, "foo"},
+		token{tokenEqual, "="},
+		token{tokenString, "hello ♥"},
+		token{tokenEOF, ""},
+	})
+}