Expand template markup to allow for conditional blocks

[Frencil]

As described by @pjvandehaar it would sometimes be useful, when available fields may vary from element to element in a single data layer, to write the general tool tip markup such that certain sections would only be displayed if the field is present on the element.

Note that handlebars.js, which was the inspiration for the double-curly markup notation in LocusZoom templates, does have a convention for conditionals, among other things.

[pjvandehaar]

For reference:

• handlebars: {{#if x}}...{{/if}} {{#each xs}}{{this}}{{/each}}
• mustache: {{#x}}...{{/x}} {{#xs}}{{a}}{{b}}{{/xs}}

[pjvandehaar]

PR https://github.com/statgen/locuszoom/issues/91

[Frencil]

Implemented on v0.5.6