feat: add the ability to load plugins in config#294

Adds the ability to specify plugins as a list of entries in a config file. This is an alternative to the current method of supplying them as command line flags, and is written to reflect the idea posted in #251 of allowing this syntax: ```yaml - plugins: - dotbot-template - dotbot-plugins/dotbot-apt - missing-plugin ``` Given that list, and assuming only the first two plugins have been added locally, and that `template` was configured as such: ```yaml - template: - ~/.gnupg/gpg-agent.conf: source_file: gpg-agent.conf params: __UNAME__: PINENTRY_PROGRAM: Darwin: /usr/local/bin/pinentry-mac Linux: /usr/bin/pinentry-gnome3 ``` The output when installed would contain this: ``` Loading plugin from dotbot-template Loading plugin from dotbot-plugins/dotbot-apt Loading plugin from missing-plugin Failed to load plugin from missing-plugin Some commands were not successfully executed Rendering template ~/.gnupg/gpg-agent.conf ```