Enable support for joining strings

This PR adds an option `join_strings` to enable the `Bibliography.join` function to be called. Currently a scenario such as:

    @string{ ruby = "Ruby" }
    @book{somebook, title="The " # ruby # " Programming Language"}

gives an output

    "The " # "Ruby" # " Programming Language"

Enabling the `join_strings` option (default is `true`) produces

    The Ruby Programming Language

which is probably what is more useful. Feature file is included, including a couple of tests for `replace_strings`, hopefully it all passes this time!