IndexError list index out of range exception when creating StringSubstitutionFilter

I'm getting an IndexError: list index out of range exception when creating a StringSubstitutionFilter. Stack trace:
```
File "/usr/src/app/etl/tasks.py", line 66, in stetl_task
etl.run()
File "/usr/local/lib/python3.7/dist-packages/Stetl-2.1.dev0-py3.7.egg/stetl/etl.py", line 154, in run
chain.assemble()
File "/usr/local/lib/python3.7/dist-packages/Stetl-2.1.dev0-py3.7.egg/stetl/chain.py", line 87, in assemble
etl_comp = factory.create_obj(self.config_dict, etl_section_name.strip())
File "/usr/local/lib/python3.7/dist-packages/Stetl-2.1.dev0-py3.7.egg/stetl/factory.py", line 28, in create_obj
raise e
File "/usr/local/lib/python3.7/dist-packages/Stetl-2.1.dev0-py3.7.egg/stetl/factory.py", line 25, in create_obj
class_obj_inst = self.new_instance(class_obj, configdict, section)
File "/usr/local/lib/python3.7/dist-packages/Stetl-2.1.dev0-py3.7.egg/stetl/factory.py", line 62, in new_instance
return class_obj(configdict, section)
File "/usr/local/lib/python3.7/dist-packages/Stetl-2.1.dev0-py3.7.egg/stetl/filters/stringfilter.py", line 62, in __init__
self.format_args_dict = Util.string_to_dict(self.format_args, self.separator)
File "/usr/local/lib/python3.7/dist-packages/Stetl-2.1.dev0-py3.7.egg/stetl/util.py", line 112, in string_to_dict
x[1] = x[1].replace(space, ' ')
IndexError: list index out of range
```

This happens when the config file contains placeholders which are passed through the command line and when the value contains spaces which are represented with tildes. Example: `stetl -c blah.cfg -a myvalue=contains~space`

Previously, as a workaround, I passed those values as environment variables, like `export STETL_myvalue=contains~space`. Then this error doesn't occur.

As you can see, this occurs in Stetl version 2.1-dev, but this also happened before the 2.0 versions.

During debugging, I found out that after I create the ETL object (`etl = ETL(vars(args_parsed), args_parsed.config_args)`) and show the config_dict, the relevant section is shown like this:
```
[my_filter]
class = stetl.filters.stringfilter.StringSubstitutionFilter
format_args = myvalue:contains space
```
It is clear that the tilde is replaced by a space earlier in the process, at the creation of the ETL object.

So, when this is passed to string_to_dict, the dict_arr will look like this:`[['myvalue','contains'],['space']]`, which obviously causes the IndexError, since the second array only contains one element.

I haven't looked yet where this error exactly occurs. This must happen after extra arguments are passed through -a, but not when arguments are passed as environment variables with the 'STETL_'-prefix.