Avoid hardcoding pip cache directory

Hardcoding the cache values in 221954d (i.e., `--download-cache
$VIRTUALFISH_HOME/_pip_cache`) will likely override the user's pip
configuration settings. In my case, I have the following in
`config.fish`:

    set -x PIP_DOWNLOAD_CACHE "$HOME/.pip/cache"

If I already have a metric ton of cached packages in
`$HOME/.pip/cache` (and I do), why would I want or need another
cache created in `$VIRTUALFISH_HOME/_pip_cache`?

(In case you're wondering, "Maybe Virtualfish should look for these
environment variables, and if they aren't found, fall back to
defaults such as `$VIRTUALFISH_HOME/_pip_cache`?" ... that would
still potentially cause problems because pip configuration can also
be stored in `~/.pip/pip.conf`.)

This commit removes the hardcoded values and instead provides
instructions in the README that explain how to use pip's cache and wheel
settings to speed up global requirements installations.
Add events for virtualenv activation. Closes #9.
Separate global-requirements functionality out into a plugin
Merge branch 'pip-config' of https://github.com/justinmayer/virtualfish

Conflicts:
	virtual.fish