Fix issue 47: enable custom batch templates

[dennisvang]

fixes #47

In addition to the existing install function override, this PR adds the ability for users to supply a custom batch template (for windows), by passing a batch_template argument to Client.download_and_install_update().

The default template WIN_BATCH_TEMPLATE can be used as an example (see here).

The following default variables are available for use in the custom template, but they can also be omitted:

• {log_lines} inserts lines that cause batch file output to be logged to file
• {src_dir} inserts path to source directory
• {dst_dir} inserts path to destination directory
• {robocopy_options} inserts robocopy options (default or custom)
• {delete_self} inserts a line that causes the batch file to delete itself (should only be used at end of file)

The user can also introduce custom template variables, e.g. {my_variable}, or they can pass a pre-filled template without any variables.

If custom template variables are used, they must be passed into Client.download_and_install_update() using the batch_template_extra_kwargs argument.

Usage example (note this template doesn't do anything useful):

custom_template = """
echo {custom_content}> "{custom_file_path}"
{delete_self}
"""
custom_template_variables = dict(
    custom_content='some content',
    custom_file_path=r'some\file\path',
)

...

client.download_and_install_update(
    ..., 
    batch_template=custom_template, 
    batch_template_extra_kwargs=custom_template_variables, 
    ...,
)

...

Also see example in tests.