Add a pretty-printer package

[eaburns]

The peg package has a pretty printer. However, I can think of a few other useful ways to pretty-print a tree. It may be good to add a pretty printer package:

/*
text_eof
    ├──────┐
    │      ├──COI["coi"]
    │      ├──KOhA["do"]
    │      └──DOhU_elidible["DOhU"]
    └──────┐
           ├────────┐
           │        ├──────KOhA["ma"]
           │        ├──────CU_elidible["CU"]
           │        └──────GOhA["mo"]
           └──IAU_elidible["IAU"]
*/
prettyprint.Tree(...)

/*
([coi do DOhU] [{ma CU mo} IAU])
*/
prettyprint.Braces(...)

/* --- The current pretty printer --- */
prettyprint.Structs(...)

Basically, take the code from here: https://github.com/eaburns/johaus/blob/master/pretty/pretty.go.