#28 Add sites dynamically

Ability to add download sites on the fly without having to update the whole application.

For example with a config file (.ini style):

[global]
url=https://test.url
path=/search
query=search=%s
method=POST

[layout]
container=div#content>div.list

I remember working on this but due to the required high variety and dynamics of download sites, this was halted.

Work of this can be found in the dynamic_sites branch