Allow racc cmdline to read from stdin if no path specified. #76

Job	Run time	Billable time
ruby-versions / Generate Ruby versions	4s	0s
test (2.5, ubuntu-latest)	25s	0s
test (2.5, macos-latest)	55s	0s
test (2.5, windows-latest)	2m 13s	0s
test (2.6, ubuntu-latest)	26s	0s
test (2.6, macos-latest)	1m 37s	0s
test (2.6, windows-latest)	2m 47s	0s
test (2.7, ubuntu-latest)	27s	0s
test (2.7, macos-latest)	1m 2s	0s
test (2.7, windows-latest)	1m 49s	0s
test (3.0, ubuntu-latest)	14s	0s
test (3.0, macos-latest)	24s	0s
test (3.0, windows-latest)	1m 11s	0s
test (3.1, ubuntu-latest)	22s	0s
test (3.1, macos-latest)	26s	0s
test (3.1, windows-latest)	1m 24s	0s
test (3.2, ubuntu-latest)	16s	0s
test (3.2, macos-latest)	26s	0s
test (3.2, windows-latest)	1m 55s	0s
test (3.3, ubuntu-latest)	16s	0s
test (3.3, macos-latest)	27s	0s
test (3.3, windows-latest)	2m 5s	0s
test (head, ubuntu-latest)	16s	0s
test (head, macos-latest)	28s	0s
test (head, windows-latest)	1m 8s	0s
test (jruby, ubuntu-latest)	4m 23s	0s
test (jruby-head, ubuntu-latest)	4m 19s	0s
test (truffleruby, ubuntu-latest)	1m 37s	0s
test (truffleruby-head, ubuntu-latest)	1m 51s	0s
	35m 13s	0s

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