Adds survivor machete.

As suggested by @John-Candlebury here:
http://smf.cataclysmdda.com/index.php?topic=8444.msg194374#msg194374.

An improved version of the machete, the survivor machete is an
autolearned craftable improvement that adds a couple points of damage
and a few techniques to the weapon.

Can occasionally spawn in survivor zed loot.
Derp.
Grammar.
Secondary skill.
Update armor.json

Adds dusters. They're modeled after the trenchcoat items but also cover the legs. Their volume, weight, and components needed to craft are 35% higher than trenchcoats.
Update recipe_armor.json

The duster recipes. Have a 35% higher component requirement than trenchcoats.
Update item_groups.json

Simply adds duster spawn chances to the same places trenchcoats spawn, and at the same spawn rates.
Fix TypeError from silly use of None defaults

Repeat after me: None is not iterable. I shall not attempt to iterate
None. I shall not pass None as the default value for a missing
dictionary key unless I want a TypeError.
Don't leave files open. It's what `with` is for.
Small mapgen fix
Added an option in the martial arts menu to prevent accidental wielding of items when fighting unarmed. (keep hands free)
Update armor.json

Adds dusters.

Removed water/rain proof from cloth dusters.
Update item_groups.json

Halved trenchcoat and duster spawns, to make up for the addition of the dusters.
Merge pull request #10177 from Mshock777/mapgen

Small mapgen fix
Merge pull request #10176 from zaroth/hands_free

Added an option in the martial arts menu to prevent accidental wielding ...
Merge pull request #10173 from narc0tiq/wtf-lang-extractor

Fix some silly Python issues with lang/extract_json_strings.py.
Merge pull request #10172 from Dragoonseal/master

Adds dusters
Herp derp.
Merge pull request #10170 from Rivet-the-Zombie/survivor-machete

Adds survivor machete.
Fixes some missing spoilage timers.

Adds spoilage timers to chicken noodle soup and sap soup.