Disable autosuggest if buffer is too large

[lbolla]

Make buffer max size configurable, defaulted to infinity ($ZSH_BUFFER_MAX_SIZE).

[z0rc]

Please document the new configuration variable. Also this variable should be under ZSH_AUTOSUGGEST_ "namespace" for consistency with other configuration variables.

[z0rc]

Can you also update https://github.com/zsh-users/zsh-autosuggestions/blob/develop/README.md#configuration with new configuration variable? This will make sure that the regular user knows about this option without checking the source code.

[z0rc]

Thank you very much! @ericfreese gentle ping.

[ericfreese]

Thank you @lbolla and @z0rc for your work here!