Support running ./bin/html2haml fully

[aprescott]

To avoid going through a gem build and gem install ./html2haml-x-y.gem process, it's useful to run ./bin/html2haml and have it automatically use lib/.

First, remove any html2haml gems with gem uninstall. Then try to use ./bin/html2haml.

$ ./bin/html2haml --version

$ touch /tmp/blah.html
$ ./bin/html2haml /tmp/blah.html
/Users/adamprescott/.rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:114:in `require': cannot load such file -- html2haml/html (LoadError)
    from /Users/adamprescott/.rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:114:in `require'
    from /private/tmp/html2haml/lib/html2haml.rb:7:in `<top (required)>'
    from /Users/adamprescott/.rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:53:in `require'
    from /Users/adamprescott/.rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:53:in `require'
    from ./bin/html2haml:3:in `<main>'

If you gem install html2haml (say, version 1.0.1), ./bin/html2haml --version will incorrectly report whatever is for lib/'s version, but will instead try to generate with the version you gem install'd. To see this, note that 1.0.1 has no --ruby19-attributes flag, then:

$ cd /tmp/html2haml # git repo
$ ./bin/html2haml --version
/Users/adamprescott/.rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:114:in `require': cannot load such file -- html2haml/html (LoadError)
[...]

$ cd
$ gem install html2haml -v '1.0.1'
$ cd /tmp/html2haml # git repo
$ html2haml --version
html2haml 2.0.0.beta.2
$ ./bin/html2haml --trace --ruby19-attributes
/Users/adamprescott/.gem/ruby/2.0.0/gems/html2haml-1.0.1/lib/html2haml/exec.rb:40:in `parse': invalid option: --ruby19-attributes (OptionParser::InvalidOption)
    from /Users/adamprescott/.gem/ruby/2.0.0/gems/html2haml-1.0.1/lib/html2haml/exec.rb:22:in `parse!'
    from ./bin/html2haml:7:in `<main>'

The version is reported as 2.0.0.beta.2 from the repo's value, but it's still using ~/.gem/ruby/2.0.0/gems/html2haml-1.0.1 code.

I think the fix here is to $LOAD_PATH.unshift(...) in bin/html2haml so that it always makes lib/ available to prioritise require "..." calls.