Allow configuration of typographical replacements, e.g. smart quotes

If you're living in a non-UTF-8 character set, then you can configure what quotes are smartly changed.
If my regexes are messing things up, you can also turn this off with smart_quotes: false
Also updated docs.

This closes #50

spec/args/typographer_spec.rb

@@ -44,8 +44,8 @@ describe Squib::Args::Typographer do
     expect(t.process(%{can't})).to eq(%{can\u2019t})
   end
 
-  it "single quotes inside double quotes" do
-    expect(t.process(%{"'I can't do that', he said"})).to eq(%{\u201C\u2019I can\u2019t do that\u2019, he said\u201D})
+it "single quotes inside double quotes" do
+    expect(t.process(%{"'I can't do that', he said"})).to eq(%{\u201C\u2018I can\u2019t do that\u2019, he said\u201D})
   end
 
   it "replaces the straightforward ones" do

spec/data/samples/config_text_markup.rb.txt

@@ -0,0 +1,52 @@
+cairo: antialias=(["subpixel"])
+cairo: save([])
+cairo: set_source_color([:black])
+cairo: translate([10, 10])
+cairo: rotate([0])
+cairo: move_to([0, 0])
+pango: font_description=([MockDouble])
+pango: text=(["\"'Yaml ain't markup', he says\""])
+pango: markup=(["foo"])
+pango: width=([307200])
+pango: height=([204800])
+pango: wrap=([#<Pango::Layout::WrapMode word-char>])
+pango: ellipsize=([#<Pango::Layout::EllipsizeMode end>])
+pango: alignment=([#<Pango::Layout::Alignment left>])
+pango: justify=([false])
+pango: spacing=([0])
+cairo: update_pango_layout([MockDouble])
+cairo: move_to([0, 0])
+cairo: update_pango_layout([MockDouble])
+cairo: show_pango_layout([MockDouble])
+cairo: rounded_rectangle([0, 0, 0, 0, 0, 0])
+cairo: set_source_color([:cyan])
+cairo: set_line_width([2.0])
+cairo: stroke([])
+cairo: restore([])
+surface: write_to_png(["_output/config_text_00.png"])
+cairo: antialias=(["subpixel"])
+cairo: save([])
+cairo: set_source_color([:black])
+cairo: translate([10, 10])
+cairo: rotate([0])
+cairo: move_to([0, 0])
+pango: font_description=([MockDouble])
+pango: text=(["This has typographic sugar --- and ``explicit'' quotes --- but the quotes are \"dumb\""])
+pango: markup=(["foo"])
+pango: width=([307200])
+pango: height=([204800])
+pango: wrap=([#<Pango::Layout::WrapMode word-char>])
+pango: ellipsize=([#<Pango::Layout::EllipsizeMode end>])
+pango: alignment=([#<Pango::Layout::Alignment left>])
+pango: justify=([false])
+pango: spacing=([0])
+cairo: update_pango_layout([MockDouble])
+cairo: move_to([0, 0])
+cairo: update_pango_layout([MockDouble])
+cairo: show_pango_layout([MockDouble])
+cairo: rounded_rectangle([0, 0, 0, 0, 0, 0])
+cairo: set_source_color([:cyan])
+cairo: set_line_width([2.0])
+cairo: stroke([])
+cairo: restore([])
+surface: write_to_png(["_output/config_disable_text_00.png"])

spec/samples/samples_regression_spec.rb

@@ -51,6 +51,7 @@ describe "Squib samples" do
       basic.rb
       cairo_access.rb
       csv_import.rb
+      config_text_markup.rb
       custom_config.rb
       draw_shapes.rb
       embed_text.rb