Allow using different patterns to identify keywords into markdown

[BrunoRiibeiro]

Create a new inline command to allow using different patterns to identify keywords into the markdown document. These patterns can be:

• Non-alphanumerical. E.g. "((key))", "$key$";
• Alphanumerical. E.g. "openkeyclose";

Attention: every pattern must have one occurence of string "key". E.g. "keyopen_key_keyclose" --> this is not allowed.