enable writing of only selected output paths

- add WRITE_SELECTED setting
- add --write-selected commandline option
Add python 3.4 to tox config.
Merge pull request #1316 from saimn/tox

Add python 3.4 to tox config.
set _cache_open func even if not loading cache, fixes autoreload

The _cache_open attribute of the FileDataCacher class was not set when
settings[load_policy_key] was not True, so saving later failed.
As a precaution, replaced the `if ...: return` style with a plain
if structure to prevent such readability issues and added tests.
Merge pull request #1314 from smartass101/write-selected-only-PR

Enable writing of selected output paths only. Refs #224.
Merge pull request #1318 from smartass101/fix_cacher_open_func

set _cache_open func even if not loading cache, fixes autoreload
Handle list metadata as list of string in MarkdownReader