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

samples/config_disable_quotes.yml

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+# If we want to disable smart quoting and only allow explicit quoting within markup, 
+# use this option
+smart_quotes: false

samples/config_text_markup.rb

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+require 'squib'
+
+Squib::Deck.new(config: 'config_text_markup.yml') do
+  text str: %{"'Yaml ain't markup', he says"},
+       x: 10, y: 10, width: 300, height: 200, font: 'Serif 20',
+       markup: true, hint: :cyan
+  save_png prefix: 'config_text_'
+end
+
+Squib::Deck.new(config: 'config_disable_quotes.yml') do
+  text str: %{This has typographic sugar --- and ``explicit'' quotes --- but the quotes are "dumb"},
+       x: 10, y: 10, width: 300, height: 200, font: 'Serif 20',
+       markup: true, hint: :cyan
+  save_png prefix: 'config_disable_text_'
+end

samples/config_text_markup.yml

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+# We can configure what characters actually get replaced by quoting them with unicode code points.
+lsquote: "\u2018" #note that Yaml wants double quotes here to use escape chars
+rsquote: "\u2019"
+ldquote: "\u201C"
+rdquote: "\u201D"
+em_dash: "\u2014"
+en_dash: "\u2013"
+ellipsis: "\u2026"