Improvments and refactoring

This changes will provides an Toolbar integration that allows to either export as email or export as files into GDrive. The recursive code architecture allows to parse the whole document tree, even if table cells are formatted. Researchers greatly benefit from a equation export as LaTex. 

# Improvements
* Equation support generates latex equations and puts them into the output markdown surrounded by a $ sign.
* Toolbar integration allows an easier execution of the script.
* Recursive code architecture makes the code cleaner and readable. A document is a tree like structure. This modifications will walk over the whole document tree recursively. Now the script can handle text formats within table cells or bullet point lists.  
* Markdown documents can be exported directly as files into Google Drive. A warning is presented if files are about to get overwritten.

# Changes for discussion
* Code fencing is implemented the way GitHub flavored markdown does it.