Add support for opting out of using GitHub API

The fact that "brew search" can query both the local repository and
numerous remote repositories on GitHub via the GitHub API is convenient,
but silently sending the user's query terms to github.com without the
user's consent or even knowledge is not respectful of the user's
privacy.

This patch allows users to "opt out" of using the GitHub API altogether
by setting the HOMEBREW_GITHUB_API_OPTOUT environment variable.  The
value of the environment variable does not matter (it can even be
empty!).  My preference would be for "opt in" behavior, but
unfortunately that would break backwards compatibility.

For bash : export HOMEBREW_GITHUB_API_OPTOUT=1
For csh  : setenv HOMEBREW_GITHUB_API_OPTOUT 1