Add support for configuring colors and styles

[codeinred]

This pull request adds support for configuring colors and styles in Clap. It does this by adding a StyleSpec class that holds information about the termcolor::ColorSpecs associated with various styles. This is added as a component of the App class, and it allows users to set colors, styles, and other such aspects of Clap.

For example, I can now write the following:

use clap::{Color, IntoApp, Parser, Style};

fn main() {
    let app = Args::into_app()
        .bold(Style::Good, true)
        .bold(Style::Warning, true)
        .foreground(Style::Warning, Some(Color::Green));

    // ...

In this example, this will set text displayed in Style::Good and Style::Warning to bold, and also update the warning style color to Green.

Before Customization:

After Customization:

A note on styles

This pull request functions within the bounds of the Style enum already defined by Clap, inside src/output/fmt.rs. We may want to give the options defined in this enum more descriptive names (for example, Style::Warning would become Style::SectionHeader, which is what it's used for when printing --help). We may also want to increase the number of styles (so that there's a Warning style and a SectionHeader style)

Considerations on the API

This adds Style as part of claps public API, and it also exposes termcolor::Colorasclap::Color`.

[codeinred]

I made some changes to resolve linter errors and to squash the commits. This issue has been closed. Please see #3407 instead.