Rename misleading var 'i'
Add snippet pre-parser

This does a pre-parse on smartdown content, replacing
{{some_snippet_file_name}} with the contents of a snippet 
file from the 'snippets' folder in the smart down flow source.
Handle snippet not found errors
Snippet content stripped before insertion

Since it's generally considered good practice to leave a trailing
EOF in text files, we need to strip that off before inserting snippets
or inline snippets will always end in a paragraph break.

Since the snippet tag will already be surrounded by the appropriate
whitespace, we should just strip all leading / trailing whitespace.
Parser does not break with extra trailing newlines
Change snippet tag format to {{snippet: snippet_name}}
Add acceptence test covering snippet parsing
Merge pull request #58 from alphagov/snippet-pre-parser

Snippets