Support 'ltag' table for IETF BCP-47 language tags

Resolves #226

Doc/documentation.html

@@ -42,7 +42,7 @@ When using TTX from the command line there are a bunch of extra options, these a
 The following tables are currently supported:
 <BLOCKQUOTE><TT>
 <!-- begin table list -->
-BASE, CBDT, CBLC, CFF, COLR, CPAL, DSIG, EBDT, EBLC, FFTM, GDEF, GMAP, GPKG, GPOS, GSUB, JSTF, LTSH, MATH, META, OS/2, SING, SVG, TSI0, TSI1, TSI2, TSI3, TSI5, TSIB, TSID, TSIJ, TSIP, TSIS, TSIV, VDMX, VORG, cmap, cvt, feat, fpgm, fvar, gasp, glyf, hdmx, head, hhea, hmtx, kern, loca, maxp, name, post, prep, sbix, vhea and vmtx
+BASE, CBDT, CBLC, CFF, COLR, CPAL, DSIG, EBDT, EBLC, FFTM, GDEF, GMAP, GPKG, GPOS, GSUB, JSTF, LTSH, MATH, META, OS/2, SING, SVG, TSI0, TSI1, TSI2, TSI3, TSI5, TSIB, TSID, TSIJ, TSIP, TSIS, TSIV, VDMX, VORG, cmap, cvt, feat, fpgm, fvar, gasp, glyf, hdmx, head, hhea, hmtx, kern, loca, ltag, maxp, name, post, prep, sbix, vhea and vmtx
 <!-- end table list -->
 </TT></BLOCKQUOTE>
 Other tables are dumped as hexadecimal data.

Lib/fontTools/ttLib/tables/__init__.py

@@ -56,6 +56,7 @@ def _moduleFinderHint():
 	from . import _h_m_t_x
 	from . import _k_e_r_n
 	from . import _l_o_c_a
+	from . import _l_t_a_g
 	from . import _m_a_x_p
 	from . import _n_a_m_e
 	from . import _p_o_s_t

Lib/fontTools/ttLib/tables/_l_t_a_g.py

@@ -0,0 +1,50 @@
+from __future__ import print_function, division, absolute_import
+from fontTools.misc.py23 import *
+from fontTools.misc.textTools import safeEval
+from . import DefaultTable
+import struct
+
+# https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6ltag.html
+
+class table__l_t_a_g(DefaultTable.DefaultTable):
+
+	def decompile(self, data, ttFont):
+		self.version, self.flags, numTags = struct.unpack(">LLL", data[:12])
+		assert self.version == 1
+		self.tags = []
+		for i in range(numTags):
+			pos = 12 + i * 4
+			offset, length = struct.unpack(">HH", data[pos:pos+4])
+			tag = data[offset:offset+length]
+			self.tags.append(tag)
+
+	def compile(self, ttFont):
+		dataList = [struct.pack(">LLL", self.version, self.flags, len(self.tags))]
+		stringPool = ""
+		for tag in self.tags:
+			offset = stringPool.find(tag)
+			if offset < 0:
+				offset = len(stringPool)
+				stringPool = stringPool + tag
+			offset = offset + 12 + len(self.tags) * 4
+			dataList.append(struct.pack(">HH", offset, len(tag)))
+		dataList.append(stringPool)
+		return bytesjoin(dataList)
+
+	def toXML(self, writer, ttFont):
+		writer.simpletag("version", value=self.version)
+		writer.newline()
+		writer.simpletag("flags", value=self.flags)
+		writer.newline()
+		for tag in self.tags:
+			writer.simpletag("LanguageTag", tag=tag)
+			writer.newline()
+
+	def fromXML(self, name, attrs, content, ttFont):
+		if not hasattr(self, "tags"):
+			self.tags = []
+		if name == "LanguageTag":
+			self.tags.append(attrs["tag"])
+		elif "value" in attrs:
+			value =  safeEval(attrs["value"])
+			setattr(self, name, value)