- tex_folder: adds the option to specify a .tex files folder. This makes easy
traversing more complicated file structures.
- interactive: adds user CLI interaction for selecting specific files. This
makes possible to avoid importing some sections (the Appendix for instance).

Various fixes:
- changes internal naming of variables to more explicit version
- improves correct use of spaces for arguments
- (functions: main, expand_file) changes to "with open(file) as filename:" pattern
- (function: is_input) makes explicit that also "include" statements are matched
- (function: main) adds documentation
- updates readme