Add hit grep subcommand

[dschwen]

Reason

It is often useful to find input files that set a certain parameter. Vanilla grep is not helpful here, as it has no concept of hit blocks and context. Just searching a parameter by its name alone is unproductive, as many objects share parameter names.

Design

The most straight forward implementation would utilize wildcard matches to match full parameter paths

hit grep '/Contact/*/mesh' *

or (with parameter value matching)

hit grep '/Constraints/*/secondary_subdomain=*primary' *

this would be very straight forward to implement, but I'd like to be able to specify constraints on the type of a block matched with a * as well e.g.

hit grep '/Constraints/*/[secondary_subdomain=*primary,type=GapConductanceConstraint]' *

or some other more intuitive syntax.

Impact

A useful tool for people changing APIs around or looking for examples of parameter usage in a large set of input files.