feat: support for custom loaders via `cond`

[lewis6991]

@wbthomason

[lewis6991]

We could take this further and make cond the only way to lazy load and just provide the existing handlers as utility functions:

local loader = require'packer.loader'

packer.add{
  {'my/plugin',
    cond = {
      loader.keys('n', 'ga'),
      loader.cmd('mycmd'),
      loader.event('CursorHold'),
      function(load_plugin)
        ... -- my custom loader
      end,
    },
  },
}

[wbthomason]

Didn't get a chance to post my review before merge (which is fine), but:

• I like the feature, impl looked good
• I'm OK with making cond the basis of all the lazy-loads - this also makes it easier to extend the set of loaders with community contributions. I think we'd want to play with the API to avoid cluttering specs with a bunch of loader.foo calls, though, which could be done either by supporting the current keys and just calling the appropriate loader creation function while processing a spec (my personal preference), or a solution based around community convention (e.g. "always have local keys = loader.keys, etc. at the top of your spec file")