Add theme installation command for Beamer presentations

[prudhomm]

Summary

Add install-theme command to download and install Beamer themes for presentations.

Motivation

Following the font installation feature (#4), users need an easy way to install Beamer themes like numpex from external repositories. This avoids bloating the package while providing a convenient way to set up presentation themes.

Implementation

New Command: install-theme

# List available themes
article-cli install-theme --list

# Install numpex theme
article-cli install-theme numpex

# Install to custom directory  
article-cli install-theme numpex --dir themes/

# Force re-download
article-cli install-theme numpex --force

# Install from custom URL
article-cli install-theme my-theme --url https://example.com/theme.zip

Features

• Download themes from configured URLs (GitHub releases, archives)
• Extract specific .sty files and directories (e.g., images/)
• Built-in support for numpex theme from presentation.template.d
• Configurable via [themes] section in config file
• Automatic detection of font/engine requirements

Files to Create/Modify

• src/article_cli/themes.py - ThemeInstaller class
• src/article_cli/config.py - Add get_themes_config()
• src/article_cli/cli.py - Add install-theme command
• tests/test_themes.py - Tests for theme installation
• README.md - Documentation

Related

• Depends on Add font installation command for XeLaTeX projects #4 (font installation)
• Related to Add Beamer presentation templates and theme support #3 (Beamer template support)