performance improvement from clvv

see https://github.com/clvv/z/commit/4f313ac39eb34ff08a41e5efafbf0d7a7ffcfcd1
fix bash tab completion.
deal with weird dirs a bit better

* quote tab expansions
* commented out code for thinking about dirs that break awk regex.
  Haven't quite figured out a good approach.
prune removed dirs from datafile

* this got lost when I tried to minimize writes to the data file
remove quotes from completions for now

closes #46
replace notdir hack

* Test entries in datafile via shell and pipe them into awk, rather than
  doing the test in awk.
revert accidental change

Accidentally set z not to resolve symlinks. I'm leaning towards making
this the default, but I'm not ready to "commit" yet (haha)
useless use of cat
use and document _Z_NO_RESOLVE_SYMLINKS variable
respect _Z_NO_PROMPT_COMMAND environment variable

For those who wish to set their PROMPT_COMMAND manually. Closes #44.
fix for gawk with regex chars in paths

Sub out regex chars in paths before comparing them. Fixes gawk and
doesn't seem to break other awks. Closes #54
Try to be clearer in installation docs
Merge branch 'master' of github.com:rupa/z
correct a bug when escaping regex characters

* "\\&" was just turning characters into literal &s, "\\\&" seems to
  work, at least on this OS/awk version.
* Added ) and ] to escaped characters.
another fix to the stupid regex escaping.

This one works in gawk as well as mawk, and let's hope it works in the
other bazillion flavors of awk as well. :/
metacharacters in directory names

* quote metachars in tab completions because bash doesn't, or at least,
  I can't figure out how to tell it to yet.
* tell zsh NOT to quote metachars in tab completions because it does, by
  default. this is irritating, as i'm sure it's better than me at it.
* do the irritating metachar quoting that's been a PITA on a copy
  of the variable, so we can return the original, because gsub modifies
  in place.

TODO: if we're going to be taking over metacharacter quoting, we ought
to do it less halfhazardly
let shells do the quoting in completions, like they should

Dug around in bash docs until I found out how to get it to quote
metachars in tab completions like zsh does by default, so that's
nice. I knew you you could do it, bash!
ZSH USERS NOTE BACKWARD COMPATIBILITY WARNING!

* z now handles precmd functionality for zsh, and no longer requires
  it to be set manually. However, this breaks backward compatibility
  with zsh.
  Existing users who use zsh should remove the precmd function they
  added when installing.
* z now respects _Z_NO_RESOLVE_SYMLINKS and _Z_NO_PROMPT_COMMAND
  variables under zsh.
* Documentation updated to reflect changes in installation instructions
ZSH USERS NOTE BACKWARD COMPATIBILITY WARNING!

Better documentation of zsh changes in README
ZSH USERS NOTE BACKWARD COMPATIBILITY WARNING!

use cool PWD functionality in zsh precmd (and avoid a subshell) at
the expense of a kind of clunky if/else. Thanks @sorin-ionescu.