Add setting global counters at runtime

[nvzqz]

Set global Counters at runtime using:

• Divan::counter
• Divan::items_count
• Divan::bytes_count
• Divan::chars_count
• --items-count N CLI arg
• --bytes-count N CLI arg
• --chars-count N CLI arg
• DIVAN_ITEMS_COUNT=N env var
• DIVAN_BYTES_COUNT=N env var
• DIVAN_CHARS_COUNT=N env var