Add WickedPdf.configure method

This methods uses the config block pattern to allow users to add config
values without risking overwriting WickedPdf.config.

README.md

@@ -42,9 +42,9 @@ You can see what flags are supported for the current version in [wkhtmltopdf's a
 If your wkhtmltopdf executable is not on your webserver's path, you can configure it in an initializer:
 
 ```ruby
-WickedPdf.config = {
-  exe_path: '/usr/local/bin/wkhtmltopdf',
-  enable_local_file_access: true
+WickedPdf.configure do |c|
+  c.exe_path = '/usr/local/bin/wkhtmltopdf',
+  c.enable_local_file_access = true
 }
 ```
 

lib/wicked_pdf.rb

@@ -24,6 +24,17 @@ class WickedPdf
 
   include Progress
 
+  def self.configure
+    config = OpenStruct.new(@@config)
+    yield config
+
+    @@config.merge! config.to_h
+  end
+
+  def self.clear_config
+    @@config = {}
+  end
+
   def initialize(wkhtmltopdf_binary_path = nil)
     @binary = Binary.new(wkhtmltopdf_binary_path, DEFAULT_BINARY_VERSION)
   end

test/unit/wicked_pdf_test.rb

@@ -7,6 +7,23 @@ def setup
     @wp = WickedPdf.new
   end
 
+  test 'should update config through .configure class method' do
+    WickedPdf.configure do |c|
+      c.test = 'foobar'
+    end
+
+    assert WickedPdf.config == { :exe_path => ENV['WKHTMLTOPDF_BIN'] || '/usr/local/bin/wkhtmltopdf', test: 'foobar' }
+  end
+
+  test 'should clear config through .clear_config class method' do
+    backup_config = WickedPdf.config
+
+    WickedPdf.clear_config
+    assert WickedPdf.config == {}
+
+    WickedPdf.config = backup_config
+  end
+
   test 'should generate PDF from html document' do
     pdf = @wp.pdf_from_string HTML_DOCUMENT
     assert pdf.start_with?('%PDF-1.4')