Tech for building templates for coverage reports

Tech is based 'bh-server-include'. 'bh-server-include' is modified
to allow derived technologies to modify code before it concatenated
with other. `bh-server-coverage` does this to instrument template code
with istanbul.

Tech does not produce actual report, it only collects data into
`__bh-coverage__` variable. Some other code is needed to convert this
data into one of supported by istanbul reports.

Pull request to `enb-bem-tmpl-specs` to produce the report is to follow.

@andrewblond @scf2k @arikon