Add pull-fetch command

This fetches the pull to a local 'review/$number' branch
for local review and more control over the review process.
pull-fetch: detect existing branch and give useful message
add command to comment and close an issue in one
abort comment nicely if no body was provided
pull-fetch make doc
issue-resolve make doc
make asserts use aborts instead of error
make git-hub-open respect the --remote option
Merge pull request #101 from dagolden/open-with-remote

Make git-hub-open respect --remote
Make this more idiomatic.

@dagolden, strings without spaces, $, or globbing chars don't need
quoting in Bash. I prefer to not quote things unless it's needed.
Merge pull request #100 from dagolden/asserts-should-abort

assert-* should use abort instead of error
Merge pull request #97 from dagolden/issue-resolve

add issue-resolve to comment and close in one
Merge pull request #80 from dagolden/pull-fetch

Add pull-fetch command
Support 2 args --parent and --base

without adding them to the OPTIONS parser generator.

We don't want to add one off args, but this is not the most elegant
place to put the code.

Ideally, some commands could ask for special args. We can do that later.
Improve the pull-request command

This commit makes pull-request DWIM to find a target repository and
branch name.

Because specifying owner/repo or branch on the command line is ambiguous
as to whether it is source or target, we remove those options and rely
on the existing --remote and --branch options, plus new options --parent
and --base.
support attaching a pull request to an issue
Prep #98 for merge to master
Add a couple blank lines.
Move `cmd/` files to `lib/git-hub.d/` and rm `cmd/`

All the files under `cmd/` were symlinked into lib anyway. No need for
this layer of abstraction.

Later, I will make a `plugin/` dir that people can just clone plugins
into and they'll work. `git-hub-irc` should be a plugin so I'll move
that as well.
Optimize code of interactive(), osx() and pipeline()

As the result of those functions will not change during the life of the
process, we map those functions to constants.