Add new checker for bro: bro.

This requires the new --parse-only cli option that will be in an
upcoming bro release.
README: minor update.
README: add anchors for FAQ items.
jsxhint can be used independently.

jsxhint uses its own jshint dep and does not require jshint to be
system-wide available. Removed the check for `jshint`.
New checker for R: lint.
New checker for R: svtools.
R svtools: disable the checker by default, for security reasons.
Merge pull request #1044 from riobard/fix/jsxhint

jsxhint can be used independently.
Always give absolute paths to syntax checkers 'hdevtools' and 'hlint'

Especially for 'hdevtools' this results into a more robust behaviour,
because 'hdevtools' starts a background process and changing the
current directory doesn't affect the current directory of the background
process.
Merge pull request #1046 from dan-t/master

Always give absolute paths to syntax checkers 'hdevtools' and 'hlint'
Manual: add a note about vim-virtualenv.
Hlint: fix column calculations.
Hdevtools: fix column calculations.  Cleanup.
New checker for haskell: scan.
Manual: minor cleanup.
New checker for plist XML: plutil.
README: clarifications for calling multiple checkers.
vimlint: a more efficient IsAvailable().
Minor cleanup.
Formatting.