Add 'add_row_if' and 'add_rows_if' methods

[Techassi]

This PR adds the ability to only add (a) row(s) when the specified predicate evaluates to true. This enables a smoother "flow" when building tables. For example:

let mut table = Table::new();
table
    .set_header(&vec!["Header1", "Header2", "Header3"])
    .add_row(&vec![
        "This is a text",
        "This is another text",
        "This is the third text",
    ]);
if predicate {
    table.add_row(vec!["", "", ""]);
}
table
    .add_row(vec!["", "", ""])
    .add_row(vec!["", "", ""]);

With the changes introduced by this PR, we could write the above code like:

let mut table = Table::new();
table
    .set_header(&vec!["Header1", "Header2", "Header3"])
    .add_row(&vec![
        "This is a text",
        "This is another text",
        "This is the third text",
    ]);
    .add_row_if(|| predicate, vec!["", "", ""])
    .add_row(vec!["", "", ""])
    .add_row(vec!["", "", ""]);

The same feature is implemented for add_rows_if. I also added some tests, which should cover the newly added feature.

[Techassi]

Is there anything blocking this PR? Is there anything I can do to push this forward @Nukesor?

[Nukesor]

Hey :)

Sorry for being so slow to respond. I'm currently super busy and I rarely find time to maintain my open source projects.

This will hopefully change in a week or so. We got some public holiday coming up this weekend, so I might get a bit of time to review a few things :)

[Nukesor]

I think this could be neat, but I don't like that the fact that the predicate doesn't take any parameters at all.
Right now, you could just accept a boolean instead of a closure and pre-compute the boolean beforehand.

What do you think about the following to make this a bit more useful:
The predicate could get the row/rows as an argument. The user could then decide whether to ignore or use that info and it would allow users to easily hide rows based on their content and respective filter functions.

[Techassi]

Yeah I was thinking the same thing, the predicate doesn't do much at the moment. I was also thinking about passing in some kind of "context". I like the idea of passing in row information! I will look into it and will update this PR accordingly.

[Techassi]

I updated the closure function signature from

Fn() -> bool

to

Fn(usize, &Vec<Row>) -> bool

Users now have access the the current row index and all previously added rows. Let me know, if these parameters make sense.

[Techassi]

Can we move forward with these changes @Nukesor?

[Techassi]

Let me know if there is anything left to add to this PR @Nukesor before merging!

[Nukesor]

Nice :)

I think I'm happy with the current state. Thanks for your contribution and sorry for taking so long 😅

[Techassi]

No worries! Love to hear that!