add option :trim_whitespace to parser

[clemensm]

Two options for disabling whitespace trimming have been added:

1. Either pass trim_whitespace: false to Eml.parse() as an option
2. For app wide override set
   config :eml, trim_whitespace: false

If both variants are used, the option passed directly to the parser will
override the app wide config.

If no option is given, true will be used as the default value and thus
keep the old behavior.

Reason for the change: I've got an app where we receive user created html (in froala) which we transform using Eml. However we sometimes got the problem that we've got spans in the html for the background color of some text, and currently Eml removes those whitespaces, something that is not expected by the end users.

So we've created this fork which simply adds the two options above, would be nice if you could merge it so that we can remove our own fork.