allow the user prelude to import Hawk libraries

closes #46
When to use and not use Hawk

When documents explicitly list the cases in which using their tools is
*not* the best match, I feel a lot more confident on the value of the
rest of the document.
Reorder, split into smaller sections, link to code
Move doc.hs to doc/README.md

To make it easier to read on github.
Text between examples
Put example closer to the text it illustrates
Hawk stands between Haskell and awk
simplify doc example

I can't believe Prelude doesn't have a version of concatMap for Monoid
instances.
fix spacing
Fix broken link.
3 Hawk modes
unindent code sections
Use the real tree example
Explain the tree example
fix getResult bug, show srcLoc is a string not an int
Simplify the postorder example
document -d and -D
Announce magic
introduce line-based input
Merge branch 'master' of github.com:gelisam/hsl