Adding support for server environment variables in deploy.rb, (:env_vars)

Users may specify environment variables, as a string, to be passed to the ssh commands.  

Idea: Our app is ported from a heroku environment.  To use nina effectively, in the 12-factor-app paradigm, we need to be able to pass values at deploy time.

```ruby
# deploy.rb

set :env_vars, "foo=1 bar=baz"

# .env stores key-value pairs
set :env_vars, { File.open(".env").readlines.map(&:chomp).join(" ") }
````