Fixes #30: String interpretation issues

Added a check for plain strings (single or double quoted).

This also resolves the following issue:

#### Before :
`{"my string.with a dot"|myFunction}`
outputs to
`<?php echo myFunction( my string["with a dot"] ); ?>`
which results in a parse error.

#### After :
`{"my string.with a dot"|myFunction}`
outputs to
`<?php echo myFunction( "my string.with a dot" ); ?>`
which is the expected behavior.