Added txt2tags reader

http://txt2tags.org/

There are two points which currently do not match the official
implementation.

1. In the official implementation lists can not be nested like the
following but the reader would interpret this as a bullet list with the
first item being a numbered list.

```
  - + This is not a list
```

2. The specification describes how URIs automatically becomes links.
Unfortunately as is often the case, their definitiong of URI is not
clear. I tried three solutions but was unsure about which to adopt.

* Using isURI from Network.URI, this matches far too many strings and is
therefore unsuitable
* Using uri from Text.Pandoc.Shared, this doesn't match all strings that
the reference implementation matches
* Try to simulate the regex which is used in the native code

I went with the third approach but it is not perfect, for example
trailing punctuation is captured in Urls.

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