Support MSVC compiler options starting with "/"

[fxb]

There already is a TODO in msvc.rs mentioning this, but I think it is good to have an issue open for better visibility.

It seems quite straight-forward to add support for options starting with / as well.

See: https://msdn.microsoft.com/en-us/library/610ecb4h.aspx

Options are specified by either a forward slash (/) or a dash (–).

Additionally, it might be good to also support options that accept arguments after a space.

If an option takes an argument, the option's description documents whether a space is allowed between the option and the arguments

For example, I have encountered defines being set like /D WIN32 instead of /DWIN32. Maybe this should have a separate issue.