If there is directory under content, do not create first page
fix page navigation error when there is no directory
add false to index option
add `extra` support in python-markdown

support markdown in html blocks
Simplify the default configuration file
optimize code
Disable installing theme to output when init site
move install_theme from init site to cli
fix bug: change all None value to empty string
add short command for generate and preview
Support Chinese category and filename
Add python shebang
Change variable name: file_path to sfile_path
Modify: get realpath of source file in cli
Support multi level directory
Modify write file method

1. Merge write_file between Page and Index Generator
2. Move it to cli.py
Remove all double-underlines in class methods
Adjust PagegGenerator class method sequence
Remove .err in tango.css
Move preview from class