Add .e as a variant of verilog for encrypted verilog - #194

I wanted to do some restructuring within Bender and the manifest to allow users to specify the type of each individual file from within the Bender.yml (even allowing to override the default recognized .v, .vhd, ...). I don't think .e will be uniquely identifiable as encrypted verilog.

Closing as functionality is now supported with v: my_file.e or similar since #207.