addons: detect more executable file extensions, such as .exe

In addition to hledger-X and hledger-X.hs, other common executable file
extensions will be recognised. In particular, on windows hledger now
lists hledger-web.exe as an add-on. The current list is:
bat, com, exe, hs, lhs, pl, py, rb, rkt, sh.
tools: make test-addons