Use one ps execution instead of multiple when possible

[stroncium]

Ok, so I examined all ps implementations I found and it turns out they have one thing in common: they print comm and args values(doesn't apply to headers) left aligned.

Using this and the fact we run ps itself with controlled arguments, we parse ps output, find the line with ps itself, find out locations of comm and args and length of comm and use this info to parse all the lines. Afterwards, we exclude ps itself from resulting process list(which was a side-effect of previous implementation and probably is the right thing to do anyway).

Fixes #14

[sindresorhus]

Can you give the PR a proper title?

[sindresorhus]

Would be good to maybe add some more tests.

[stroncium]

@sindresorhus It's quite hard to test it for good, but I've added one more test where most parameters(except mem and cpu) are controlled.

[sindresorhus]

@stroncium I upgraded ps-list to this version in fkill-cli and it changed from showing the process name to showing the full path truncated...