Add some options and fix a crash on Firefox

• Firefox: fix that Vimium C would break some iframes
• Vomnibar: fix a regression which would interrupt inputting by IME
• Vomnibar settings: add action=""/"opener" (for #198)
• add an option opener to some commands which open URLs, to always set opener
• add an option richText to some commands in order to copy HTML content
• FindMode: add a cache for 3~6 seconds, so if a page changes rapidly, shown count may be a bit inaccurate
• paste from clipboard: now accept a string of up to 20M characters
• add a command sortTabs to sort tabs, and support sort=""/create/recency (#207)
• LinkHints: add an option position and all position options accept "default" (#208)
• add a command captureTab to capture the visible area of a current tab
• now open Vimium C Options page on first installation
• add some other command options
• fix some bugs and wrong usages