Make third-party JS paths configurable

[darshan-Jahagirdar]

Fixes #42.

This makes the third-party JavaScript asset URLs in the generated HTML configurable at build time while preserving the current bundled defaults.

Changes:

• adds build configuration variables for jquery, mustache, cbuffer, bootstrap bundle, d3, and pako
• replaces hard-coded third-party script paths in index.tpl.html with template placeholders
• substitutes those placeholders during assemble-html

Verification:

• manually assembled html5-client/output/index.html with the default bundled paths and confirmed all placeholders were replaced
• manually assembled a custom-path variant using /usr/share/javascript/... style paths
• git diff --check