Add custom tags to markdown

[angrykoala]

Simple tags to use before rendering markdown.

e.g.
{{ include "file" }}

[angrykoala]

Use tags different to EJS (maybe add option to different tags parsers)

[angrykoala]

Use custom tags and 3 steps for rendering in EJS instead of direct rendering:

1. change all <% for <%% to avoid ejs injection
2. Change all custom tags for it's command in ejs (e.g. {{ include "test"}} for <%- include("test") %>
3. Execute ejs rendering

[angrykoala]

the option above require executing recursively the parsing command

[angrykoala]

Custom tags parser: https://github.com/angrykoala/xejs