Adding trim option to ERB, closes #840

The difference described in rspec spec:

```ruby
require 'rspec/autorun'
require 'erb'

describe ERB do
  let(:strip_template) { "<% if 1 == 1 -%>\nPassed<% end -%>\n" }
  let(:normal_template) { "<% if 1 == 1 %>\nPassed<% end %>\n" }
  context 'without trim mode' do
    it 'accepts normal templates' do
      template = ERB.new(normal_template)
      expect do
        template.result(binding)
      end.to_not raise_error SyntaxError
    end

    it 'does not trim newlines' do
      template = ERB.new(normal_template)
      template.result(binding).should eql "\nPassed\n"
    end

    it 'does not accept dashes at open/close tags' do
      template = ERB.new(strip_template)
      expect do
        template.result(binding)
      end.to raise_error SyntaxError
    end
  end

  context 'with trim mode' do
    it 'accepts normal template' do
      template = ERB.new(normal_template, nil, '-')

      expect do
        template.result(binding)
      end.to_not raise_error SyntaxError
    end

    it 'accepts dashes at open/close tags' do
      template = ERB.new(strip_template, nil, '-')
      expect do
        template.result(binding)
      end.to_not raise_error SyntaxError
    end

    it 'trims newlines' do
      template = ERB.new(strip_template, nil, '-')
      template.result(binding).should eql 'Passed'
    end
  end
end
```