Move filter_contents to plugin system

plugins/shell_filter_file_contents/README.md

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+## Shell Script File Filter
+
+This plugin uses shell scripts in order to perform filtering of files.
+If your preferred scripting is done via shell, this tool is for you.
+Be noted, though, that this method can cause an order of magnitude slow
+down. For small repositories, this wont be an issue.
+
+To use the plugin, add
+`--plugin shell_filter_file_contents=path/to/shell/script.sh`.
+The filter script is supplied to the plugin option after the plugin name,
+which is in turned passed to the plugin initialization. hg-fast-export
+runs the filter for each exported file, pipes its content to the filter's
+standard input, and uses the filter's standard output in place
+of the file's original content. An example use of this feature
+is to convert line endings in text files from CRLF to git's preferred LF,
+although this task is faster performed using the native plugin.
+
+The script is called with the following syntax:
+`FILTER_CONTENTS <file-path> <hg-hash> <is-binary>`
+
+```
+-- Start of crlf-filter.sh --
+#!/bin/sh
+# $1 = pathname of exported file relative to the root of the repo
+# $2 = Mercurial's hash of the file
+# $3 = "1" if Mercurial reports the file as binary, otherwise "0"
+
+if [ "$3" == "1" ]; then cat; else dos2unix; fi
+-- End of crlf-filter.sh --
+```

plugins/shell_filter_file_contents/__init__.py

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+#Pipe contents of each exported file through FILTER_CONTENTS <file-path> <hg-hash> <is-binary>"
+import subprocess
+import shlex
+import sys
+from mercurial import node
+
+def build_filter(args):
+    return Filter(args)
+
+class Filter:
+    def __init__(self, args):
+        self.filter_contents = shlex.split(args)
+
+    def file_data_filter(self,file_data):
+        d = file_data['data']
+        file_ctx = file_data['file_ctx']
+        filename = file_data['filename']
+        filter_cmd = self.filter_contents + [filename, node.hex(file_ctx.filenode()), '1' if file_ctx.isbinary() else '0']
+        try:
+            filter_proc = subprocess.Popen(filter_cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
+            d, _ = filter_proc.communicate(d)
+        except:
+            sys.stderr.write('Running filter-contents %s:\n' % filter_cmd)
+            raise
+        filter_ret = filter_proc.poll()
+        if filter_ret:
+            raise subprocess.CalledProcessError(filter_ret, filter_cmd)
+        file_data['data'] = d