add log helper with datetime to .helpers-rc
factor log-parsing, pretty-printing logic from glg
refactor Piece; support bolding “tag” refnames
fix two fix-broken-links bugs

- exit with error when 0 candidates are found
- `rm -f` an existing link before attempting to overwrite it; otherwise,
  if it exists and points to a directory, the overwrite may just create
  a new link in the destination directory.
add `git tags` / `g t` helper

uses fancy table-pretty-print logic from piece.py.

fixes #29