Clarify adding package in configuration layer
Skip installation of nonexistent packages

When attempting to install a package on startup, test whether it is in
the package archives list. If it is not, skip the installation attempt
and issue a warning.

Resolves #212
Integrate ahs with evil search functionality

When auto-highlight-symbol is used, it does not currently place the
searched term into the search ring, so that after ahs mode is left,
hitting "n" or "N" does not return to the search term that was selected
by hitting "*" or "#".  Some extra steps must be taken to perform the
expected search behavior of "n" and "N" after "*" or "#".

This also integrates the searched term into the ex mode substitute
command, so that "%s//replace/" works as expected.