[pelican_comment_system] Fixes wrongly reinitialization, updates requirements list and feeds can now be disabled

Fix:
 - Fixed wrongly reinitialization in combination with the upcoming redesign of the i18n_subsite plugin
 - Added pelican 3.4 to the requirements list.

Feature:
 - Comment feeds can now be disabled
Merge pull request #223 from Scheirle/pcs_i18n_fix

[pelican_comment_system] Fixed wrongly reinitialization + updated requirements list.