Added functions to clone git submodules when they are present

[jonkeane]

install_git() now has the option submodules. When it is submodules = TRUE the submodules that are listed in the .gitmodules file are cloned before the package is installed.

There were a number of functions that needed to be added for parsing the .gitmodule files, making sure ssh request urls are translated to https request urls. This does not (yet) include the submodule option for install_github(). Although I'm happy to add it, if this approach works for the authors. I've written tests for the one function that has no side effects.

This addresses #1163

[rtobar]

I have an updated version of this patch on top of the current master, plus a small improvement. Should I open a new pull request for it? Otherwise, how can I contribute to this one?

[hadley]

Would you be mind porting this to remotes? That's what devtools will use for installation in the future. We are working on breaking devtools up into smaller pieces to try and make development a bit more fluid.

[bschilder]

@jonkeane Has the submodules= arg since been removed?

devtools::install_git("https://github.com/RajLabMSSM/echofinemap.git", submodules=TRUE)
devtools::install_github("RajLabMSSM/echofinemap", submodules=TRUE)

Warning message:
Arguments in `...` must be used.
✖ Problematic argument:
• submodules = TRUE