Enable trim mode with ERB

[nickhammond]

This PR enables trim_mode for blank lines with <% -%> in ERB which is what's enabled in Rails so it's fairly familiar for those that have built a Rails app. trim_mode skips creating extra blank lines when there's no output which can be helpful for more complicated envify scripts, for example with AWS secrets manager. I suppose you could move this to a lib file and move some of this setup out of here but it's also not that complicated to really justify doing that. The envify command currently fails with a syntax error if you attempt to end your ERB tags with -%>.

<%
  require 'aws-sdk-secretsmanager'
  client = Aws::SecretsManager::Client.new(region: "us-west-2")
-%>

<% client.list_secrets.secret_list.each do |secret| -%>
<% value = JSON.parse(client.get_secret_value(secret_id: secret.arn).secret_string) -%>

<% value.each_pair do |k,v| -%>
<%= k %>=<%= v %>
<% end -%>
<% end -%>