Allow for other text encodings besides UTF8

Current version of html-conduit assumes utf8 encoding.
I've added a set of _with functions to allow other encodings from Data.Text.Encoding.

This still suffers from the following problems:
* Data.Text.Encoding only really supports UTF variants and Latin1 (my particular use case)
* Untested with Data.Text.ICU.Convert (but it should work)
* What would really be great would be to decode the head, find the enconding from the 
```<meta charset="???">``` field, and then apply it when decoding everything.

Perhaps when I have some more time.
Cheers