Added option -p to read graph data from stdin/pipe
README.md updated according to dc1e4fb.
Pipe/stdin option [-p] now compatible with grep option [-g PATTERN]
Fixed ambiguity for [-g PATTERN] is used when both [-p] and [::: [FILE]…] are present.

[::: [FILE]…] always override [-p]
Fixed error on some older versions of gnuplot where GPVAL_TERMINALS is not defined.
Merge pull request #7 from ch41rmn/master

Plot data from stdin using -p