Added --resource-path=SEARCHPATH command line option.

SEARCHPATH is separated by the usual character,
depending on OS (: on unix, ; on windows).

Note: This does not yet work for PDF output, because the
routine that creates PDFs runs outside PandocMonad.
(This has to do with its use of inTemporaryDirectory and
its interaction with our exceptions.)

The best solution would be to figure out how to move the
PDF creation routines into PandocMonad.  Second-best,
just pass an extra parameter in?

See #852.

MANUAL.txt

@@ -690,6 +690,12 @@ General writer options
     repeatedly to include multiple files. They will be included in the
     order specified.  Implies `--standalone`.
 
+`--resource-path=`*SEARCHPATH*
+
+:   List of paths to search for images and other resources.
+    The paths should be separated by `:` on linux, unix, and
+    MacOS systems, and by `;` on Windows.
+
 Options affecting specific writers
 ----------------------------------
 

src/Text/Pandoc/App.hs

@@ -70,7 +70,7 @@ import System.IO.Error (isDoesNotExistError)
 import Text.Pandoc
 import Text.Pandoc.Builder (setMeta)
 import Text.Pandoc.Class (PandocIO, getLog, withMediaBag,
-                          extractMedia, fillMediaBag)
+                          extractMedia, fillMediaBag, setResourcePath)
 import Text.Pandoc.Highlighting (highlightingStyles)
 import Text.Pandoc.Lua ( runLuaFilter )
 import Text.Pandoc.PDF (makePDF)
@@ -414,6 +414,7 @@ convertWithOpts opts = do
   let eol = fromMaybe nativeNewline $ optEol opts
 
   runIO' $ do
+    setResourcePath $ "." : (optResourcePath opts)
     (doc, media) <- withMediaBag $ sourceToDoc sources >>=
               (   (if isJust (optExtractMedia opts)
                       then fillMediaBag (writerSourceURL writerOptions)
@@ -569,6 +570,7 @@ data Opt = Opt
     , optIncludeBeforeBody     :: [FilePath]       -- ^ Files to include before
     , optIncludeAfterBody      :: [FilePath]       -- ^ Files to include after body
     , optIncludeInHeader       :: [FilePath]       -- ^ Files to include in header
+    , optResourcePath          :: [FilePath] -- ^ Path to search for images etc
     , optEol                   :: Maybe Newline    -- ^ Enforce line-endings
     }
 
@@ -638,6 +640,7 @@ defaultOpts = Opt
     , optIncludeBeforeBody     = []
     , optIncludeAfterBody      = []
     , optIncludeInHeader       = []
+    , optResourcePath          = []
     , optEol                   = Nothing
     }
 
@@ -1052,6 +1055,14 @@ options =
                   "FILE")
                  "" -- "File to include after document body"
 
+    , Option "" ["resource-path"]
+                (ReqArg
+                  (\arg opt -> return opt { optResourcePath =
+                                   splitSearchPath arg })
+                   "SEARCHPATH")
+                  "" -- "Paths to search for images and other resources"
+
+
     , Option "" ["self-contained"]
                  (NoArg
                   (\opt -> return opt { optSelfContained = True,