Allowed exponential notation when parsing numbers, closes #173

```
⧐  1e-10
0.0000000001
⧐  '1e-10'.json()
0.0000000001
```

evaluator/evaluator_test.go

@@ -48,6 +48,9 @@ func TestEvalNumberExpression(t *testing.T) {
 		input    string
 		expected float64
 	}{
+		{"1e1", 10},
+		{"1e-1", 0.1},
+		{"1e+1", 10},
 		{"5.5", 5.5},
 		{"1.1 + 2.1", 3.2},
 		{"5.5 + 2.2", 7.7},

lexer/lexer.go

@@ -346,17 +346,40 @@ func (l *Lexer) readIdentifier() string {
 	return l.input[position:l.position]
 }
 
+// 12
+// 12.2
+// 12e-1
+// 12e+1
+// 12e1
 func (l *Lexer) readNumber() (number string, kind token.TokenType) {
 	position := l.position
 	kind = token.NUMBER
 	hasDot := false
+	hasExponent := false
+
+	// List of character that can appear in a "number"
+	for isDigit(l.ch) || l.ch == '.' || l.ch == '+' || l.ch == '-' || l.ch == 'e' {
+		// If we have a plus / minus but there was no exponent
+		// in this number, it means we're at the end of the
+		// number and we're at an addition / subtraction.
+		if (l.ch == '+' || l.ch == '-') && !hasExponent {
+			return l.input[position:l.position], kind
+		}
+
+		// If the number contains as 'e',
+		// we're using scientific notation
+		if l.ch == 'e' {
+			hasExponent = true
+		}
 
-	for isDigit(l.ch) || l.ch == '.' {
+		// If we have a dot, let's check whether this is a range
+		// or maybe a method call (122.string())
 		if l.ch == '.' && (l.peekChar() == '.' || !isDigit(l.peekChar())) {
 			return l.input[position:l.position], token.NUMBER
 		}
 
 		if l.ch == '.' {
+			// If we have 2 dots in a number, there's a problem
 			if hasDot {
 				return "", token.ILLEGAL
 			}
@@ -367,6 +390,12 @@ func (l *Lexer) readNumber() (number string, kind token.TokenType) {
 		l.readChar()
 	}
 
+	// If the number ends with the exponent,
+	// there's a problem.
+	if l.input[l.position-1] == 'e' {
+		return "", token.ILLEGAL
+	}
+
 	return l.input[position:l.position], kind
 }
 

lexer/lexer_test.go

@@ -76,6 +76,11 @@ nullo
 &^>><<
 $111
 '123'
+12+12
+12e10
+12e+10
+12e-10
+12e
 `
 
 	tests := []struct {
@@ -282,6 +287,13 @@ $111
 		{token.BIT_LSHIFT, "<<"},
 		{token.ILLEGAL, "$111"},
 		{token.STRING, "123"},
+		{token.NUMBER, "12"},
+		{token.PLUS, "+"},
+		{token.NUMBER, "12"},
+		{token.NUMBER, "12e10"},
+		{token.NUMBER, "12e+10"},
+		{token.NUMBER, "12e-10"},
+		{token.ILLEGAL, ""},
 		{token.EOF, ""},
 	}