Option to render human-friendly tables

When rendering a markdown table, this crate strips all the padding whitespace in table cells and sets the header rule to the same width as the respective header:

```markdown
|Here is a header|Another header|
|:---------------|:-------------|
|foo|bar|
|quux|baz|
|something really long||
```

This is valid markdown for representing the table, but it's not very human-friendly. It would be nice if there were an option to have cell whitespace padding and for each column to have a uniform width (i.e., the maximum width of the cells in that column + padding). So we'd get output like this:

```markdown
| Here is a header | Another header |
| :-------------------- | :------------- |
| foo | bar |
| quux | baz |
| something really long | |
```

These tables are equivalent, but the latter is much easier to read.