Add height variables
Fix inheritance of `markdown-header-face'

Do not inherit it from `alect-title' as it is used only for single
headers (without levels).
Modify info-title faces

Info titles may look small if they are inherited from `alect-title-N'
faces and `alect-multiple-titles-height' is set to 1.0.
Add description of height variables
Update version (0.3.6)