support fish shell in client configure command

these changes add support for the fish shell in the client configure
command. the main additions are a printer for exporting enviroment
variables (opamConfigcommand:print_fish_env) and a modification to the
dot file finder heuristic (opamMisc:guess_dot_profile).
Merge pull request #655 from 94c3/fish_shell_support

support fish shell in client configure command