Add local-test shell script, which I use on the target

[cxw42]

Overview

Problem: I work on lots of different targets with different configurations. Not all targets have ~/.gstreamer-1.0/plugins in GST_PLUGIN_PATH.

Solution proposed by this PR: Add a shell script that:

1. installs gst-perf in a temporary directory; and
2. opens a shell with that directory added to GST_PLUGIN_PATH.

I have been using this approach for several months on other projects and have had good success with it.

Usage example

$ ./autogen.sh && ./configure
$ ./test-locally.sh
# ... messages as it installs into ./test-locally-install-dir
make[1]: Leaving directory '.../gst-perf'
>>>  bash
$ gst-launch-1.0 -v videotestsrc ! 'video/x-raw,framerate=30/1' ! perf ! fakesink sync=true
# ... pipeline runs, using the local perf

Other

I added an EditorConfig file that supported editors will use to apply the project's indentation settings, as I understand them.

Thank you for considering this PR!