nimble packages should standardize on a build directory for generated files, to
keep git status clean
## proposal
* nimble init should create a build dir, in addition to what it already
creates (src, tests, foo.nimble)
* it should also add a .gitignore file with contents:
/build
Run
* the default template code it generates should write foo.nim to build/foo
instead of foo ; likewise with generated binaries for nim test
## benefits
* makes git status clean by default (which is a pain when using posix where
binaries have no extension; there are workarounds but they're fragile eg with
other files that are (non-nim) source or documentation/config files yet don't
contain extensions)
* easy to remove build folder completely and safely
