styleTweaks: Fix missing this.head issue

When nightMode (or another module) that loads before styleTweaks calls
enableSubredditStyle or disableSubredditStyle, this.head is undefined.
Merge pull request #1311 from mc10/style-tweaks

styleTweaks: Fix missing this.head issue
code cleanup
added conserveMemory option to hide offscreen images in order to free memory from the browser. especially effective for heavy users of the 'view images' button. fixes #1303
added the notion of dependent options - options that are only visible if another option is true
adjusted usertagger's call to RESUtils.toggleButton due to a change in signature
added bufferScreens option to showImages
made use of dependent options in night mode module
made use of dependent options in betteReddit module
make use of option dependency in style tweaks module
make use of option dependency in user tagger module
Fix for color input in table option
Merge pull request #1312 from matheod/colorInputTable

Fix for color input in table option