This commit adds the ability to add extra columns to the markdown tables
generated by the dump_markdown_results_table visualization. The
following is taken from the visualization's documentation:

    'extra_colums' can be an empty dict. The sample configuration below assumes
    that each benchmark result has a 'success' and 'runtime' metric for both
    variants, 'variant_1' and 'variant_2'. It adds a 'ratio' column to the table
    for the 'runtime' metric, and a 'change' column to the table for the
    'success' metric. The 'text' lambda is called once for each benchmark. The
    'text' lambda accepts a single argument---a dict---that maps variant
    names to the value of that variant for a particular metric. The lambda
    returns a string that is rendered in the benchmark's row in the new column.
    This allows you to emit arbitrary text or markdown formatting in response to
    particular combinations of values for different variants, such as
    regressions or performance improvements.

    Sample configuration:

    ```
    visualize:
    - type: dump_markdown_results_table
      out_file: "-"
      extra_columns:
        runtime:
        - column_name: ratio
          text: >
            lambda b: str(b["variant_2"]/b["variant_1"])
            if b["variant_2"] < (1.5 * b["variant_1"])
            else "**" + str(b["variant_2"]/b["variant_1"])
        success:
        - column_name: change
          text: >
            lambda b: "" if b["variant_2"] == b["variant_1"]
            else "newly passing" if b["variant_2"]
            else "regressed"
    ```

    Example output:

    ```
    ## runtime

    | Benchmark |  variant_1 | variant_2 | ratio |
    | --- | --- | --- | --- |
    | bench_1 | 5 | 10 | **2.0** |
    | bench_2 | 10 | 5 | 0.5 |

    ## success

    | Benchmark |  variant_1 | variant_2 | notes |
    | --- | --- | --- | --- |
    | bench_1 | True | True |  |
    | bench_2 | True | False | regressed |
    | bench_3 | False | True | newly passing |
    ```