Create content for LFE Hyperpolyglot

[oubiwann]

The following sections need to be created and filled in for the site http://lfex.github.io/hyperpolyglot/:

• grammar and execution
• variables and expressions
• arithmetic and logic
• strings (has LFE; missing Erlang)
• regular expressions (has LFE; missing Erlang)
• dates and time
• lists (has LFE; missing Erlang)
• fixed-length arrays
• dictionaries
• user-defined types (has LFE; missing Erlang)
• functions
• execution control
• exceptions
• streams
• emacs buffers
• files
• directories
• processes and environment
• libraries and namespaces
• objects
• lisp macros
• reflection
• java interoperation