Respect DPI settings in text, add pt unit

This has the compatibility impact that all font sizes should be divided by 3 to maintain their same sizes.

Based on PR #225. Thanks @felixleong!!

Conflicts:
	bin/squib
	lib/squib/graphics/text.rb

spec/args/unit_conversion_spec.rb

@@ -15,6 +15,11 @@ describe Squib::Args::UnitConversion do
     expect(subject.parse('1 in')).to eq(300)
   end
 
+  it 'does pt' do
+    expect(subject.parse('1pt')).to eq(4.166666666666667)
+    expect(subject.parse('1pt  ')).to eq(4.166666666666667)
+  end
+
   it 'does cm' do
     expect(subject.parse('1cm')).to eq(118.1102361)
     expect(subject.parse('1cm  ')).to eq(118.1102361)