Append current directory to the Python sys.path

[jredrejo]

Append current directory to the Python sys.path to allow using plugins that have not been installed using pypi

Closes #56

Description

According to https://github.com/zalando-incubator/Transformer/blob/master/docs/Using-plugins.rst, together with https://transformer.readthedocs.io/en/latest/Writing-plugins.html#name-resolution , when using the cli,
transformer -p mod.sub har/ >loc.py should work for a plugin called sub.py located in the mod directory.
However this didn't work unless the plugin has not been installed in the Python sys.path. Plugins left in the directory where transformer is executed are not found. Adding the current dir to the sys path fixes it.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
  Adds the current path to the sys.path where Transformer can look for plugins

Review

1. Anywhere in your system, creates a dummy.py plugin file
   2- Run Transformer with the -p python_path argument
2. See error before appying this PR and see there's no error after this PR is applied

After this PR

Document at https://transformer.readthedocs.io/en/latest/Writing-plugins.html#name-resolution that this solves the problem when the plugin is in the current directory but not if it's somewhere else.

[zincr]

🤖 zincr found 0 problems , 0 warnings

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