[nimgrep] add option --oneline to keep file:line info when piping through grep

[timotheecour]

partially addresses #7428

eg usage:

./bin/nimgrep '\bfields\b' --ext:nim --recursive --oneline | grep let
/Users/timothee/git_clone/nim/Nim/tests/js/tmangle.nim:69: # Test codegen for fields with uppercase letters:
/Users/timothee/git_clone/nim/Nim/tests/niminaction/Chapter9/configurator/configurator.nim:63:   let identDefs = toIdentDefs(fields)
/Users/timothee/git_clone/nim/Nim/tests/vm/meta.nim:174:   let fields = typedef.fields.render()
/Users/timothee/git_clone/nim/Nim/tests/vm/meta.nim:174:   let fields = typedef.fields.render()
/Users/timothee/git_clone/nim/Nim/lib/core/macros.nim:947:   let enumBody = newNimNode(nnkEnumTy).add(newEmptyNode()).add(fields)
/Users/timothee/git_clone/nim/Nim/compiler/semobjconstr.nim:148:       let fields = branch[branch.len - 1]
/Users/timothee/git_clone/nim/Nim/compiler/semobjconstr.nim:152:       let fields = branchNode[branchNode.len - 1]
/Users/timothee/git_clone/nim/Nim/compiler/semobjconstr.nim:181:         let fields = fieldsPresentInBranch(selectedBranch)
/Users/timothee/git_clone/nim/Nim/compiler/semobjconstr.nim:191:           let fields = fieldsPresentInBranch(i)

[Araq]

Thank you.