feat(txt): shortened and combined attributes

The numSentences and numParagraphs attributes have been abolished in
favor of the simpler `#p#s` format provided to the phTxt attribute
directly. This has the added value that the number of sentences per
paragraph can now be customized.

Closes #4.
Merge pull request #10 from joshdmiller/txt-combine-attrs

feat(txt): shortened and combined attributes

The numSentences and numParagraphs attributes have been abolished in
favor of the simpler `#p#s` format provided to the phTxt attribute
directly. This has the added value that the number of sentences per
paragraph can now be customized.

Closes #4.