Adding a more sensitive find & replace.

Just scratching my own itch here, so I won't feel offended if you reject the pull request. :-)

I added an `insensitive` parameter for the plugin, allowing the user to toggle a case sensitive / insensitive find & replace. I also modified the plugin to allow `find` to run when a `replace` string is absent. My intention is for the standalone find function to be used within a conditional:

    {if '{exp:streeng find="this"}{string}{/exp:streeng}'}
        We found 'this' in {string}!
    {/if}

Caveat: I've tested this on a local install of EE 2.5.5, but not extremely thoroughly.