buffer-local values for `cov-lcov-project-root`

[orzechowskid]

hi there,

#60 introduced cov-lcov-project-root as a defvar, but I often find myself working in monorepo projects where different sub-packages have different coverage files in different locations. I'd like to open a PR to define cov-lcov-project-root as buffer-local but I'm not sure what the best way is to accomplish that.

simply changing (defvar cov-lcov-project-root) to (defvar-local cov-lcov-project-root) is not sufficient, because parsing of lcov files is done in a temp buffer (in cov--read-and-parse) which needs access to that variable in order to map lcov SF paths to files on disk (in cov--lcov-parse).

the easiest thing to do would be to modify cov--read-and-parse to check the value of format and, if eq to 'lcov, set some additional buffer-local variables. another solution might be to use something like inheritenv to automatically propagate all buffer-local variables from the user's source buffer into the parser's temp buffer. there are probably other solutions too.

I figured I'd ask for advice before opening a PR, so I don't waste your time reviewing code which isn't what you'd prefer in terms of approach, so any input you had here would be welcome.

thanks!