Improve default command parsing rules

[fmbenhassine]

Currently, only --key=value pairs are supported by the default parser. This is actually a strong constraint and does not work well with command completions (one has to tap Delete=Tab to get custom completions of a given option instead of just typing Tab twice).

This issue is to relax the aforementioned constraint and support both --key=value pairs as well --key value pairs. However, the caveat is that boolean flags will make things complex and the goal is to control complexity especially in a default parser. Therefore, there will be a small constraint which is with space separated key value pairs, it is always expected to have a value for the option even if it is a boolean one, and this is necessary to be able to distinguish option values from arguments.

This should be clearly documented.