add handler for test code headers

[mariev]

add option to pass header values to markdown rendered from test code files via the test code name. include ability of user to pass a vector of headers when calling vt_file.

Example usage:

• Option 1: from the test code file

  vt_file(path, dynamic_referencing) # headers are pulled from test_that "desc" argument

• Option 2: user specified

  tc_files <- c(file1, file2, ...)
  # user-specified header overrides test code desc.
  vt_file(header = c(header1, header2, ...), tc_files, dynamic_referencing)

[mariev]

from discussion today:

use syntax:

## HEADER NAME HERE ----