Update default config behaviour

[artem-y]

In this PR:

• updated the default config behavior
• updated unit test to reflect the new expected result
• updated the README file to keep it up to date with this change

Details:

Previously, when config had omitted values, the command was replacing each of them with a default value.
For example, if the .commit.json file contained only the issue regex

{  
    "issueRegex": "ABC-[0-9]+" 
}

the resulting config would be as follows:

{  
    "issueRegex": "ABC-[0-9]+", 
    "outputIssuePrefix": "#",
    "outputIssueSuffix": "",
    "outputStringPrefix": "",
    "outputStringSuffix": ": ",
}

Note that outputIssuePrefix and outputStringSuffix have default values here that were not originally specified by the user, even though they provided the .commit.json config file.

While this was the default behavior I wanted at the time of initial development of the tool, after using it for a while and talking to other users, I think having these values set by default is counterintuitive. From now on, if the same content was provided in the config file, the missing properties will default to empty strings.

However, if no config file was specified, the tool will still default to the GitHub issue style with simple output formatting.