Add gvm_auto_{install,use} in $GVM_DIR/etc/config

Fixes #101.

These flags are consulted when gvm is asked to use an uninstalled version
or to install an unused version.  User is prompted for a response
by default but this can be overridden by adding these flags to
the confg, e.g.

$ echo gvm_auto_install=Y >> $GVM_DIR/etc/config
$ echo gvm_auto_use=Y >> $GVM_DIR/etc/config
$ gvm use groovy 1.8.8
... (installs and uses without prompting)