Prefix CSS classes, ids and custom properties

[db9795]

All classes, the clip path id and the CSS custom properties now start with maptoolkit-elevation-profile-, so generic names like .line, .marker or .section on the host page don't affect the profile.

• src/ElevationProfile.ts: PREFIX constant and a cls() helper used by the markup, the selectors and sectionValueClass(). The clip path id is now maptoolkit-elevation-profile-clip-N.
• src/style.css: selectors and custom properties renamed, values unchanged.
• README (Styling) and tests updated. A new test checks that every rendered class carries the prefix.
• Changeset minor → 1.1.0.

Custom CSS written against 1.0.0 has to use the new names, e.g. .maptoolkit-elevation-profile-line instead of .line and --maptoolkit-elevation-profile-text instead of --text.