Add support for FilePathsBase.jl

[mjram0s]

It would be useful to support the features of FilePathsBase.jl rather than requiring users to construct string paths.

julia> using FilePathsBase, JLSO

julia> x = p"foo"
p"foo"

julia> y = p"bar"
p"bar"

julia> p = x / y / "file.jlso"
p"foo/bar/file.jlso"

julia> JLSO.save(p, :baz=>1)
ERROR: MethodError: no method matching save(::PosixPath, ::Pair{Symbol,Int64})
Closest candidates are:
  save(::IO, ::Pair...; kwargs...) at /Users/maryjoramos/.julia/packages/JLSO/vWQhu/src/file_io.jl:76
  save(::IO, ::Any; kwargs...) at /Users/maryjoramos/.julia/packages/JLSO/vWQhu/src/file_io.jl:75
  save(::String, ::Any...; kwargs...) at /Users/maryjoramos/.julia/packages/JLSO/vWQhu/src/file_io.jl:77
Stacktrace:
 [1] top-level scope at REPL[26]:1