Add labels for named points on route/track

[stefanclark]

This will allow users to add labels for route/track points which have a <name></name> set.

Configure by passing point_label_options

var xxx = new L.GPX(url, {
          async: true,
          point_label_options: {
                    regexMatch: /^(POI)/i,
                    className: 'blue'
          }
        }).addTo(map)

Example CSS

div.leaflet-marker-icon.circle.blue {
        background-color: blue;
        border-color: blue;
        border-radius: 9px;
        border-style: solid;
        border-width: 1px;
        width: 15px;
        height: 15px;
        opacity: 0.6;
        white-space: nowrap;
}

div.leaflet-marker-icon.circle.blue label {
        position: relative;
        left: 15px;
        top: -15px;
        font-weight: bold;
        text-shadow: 0 0 3px white;
}

[mpetazzoni]

I've implemented a slightly more flexible version of this with marker_options.pointMatchers (see the README for details). Thanks for the suggestion and the contribution!