Cache dependencies during install macro

New usage:
```
@ BinDeps.install [:tk => :libtk, :tcl=>:libtcl]
```

Writes (to `deps/deps.jl`), e.g.:

```
const libtcl = "C:\\Users\\isaiah\\.julia\\RPMmd\\deps\\usr\\x86_64-w64-mingw32\\sys-root\\mingw\\bin\\tcl85"
const libtk = "C:\\Users\\isaiah\\.julia\\RPMmd\\deps\\usr\\x86_64-w64-mingw32\\sys-root\\mingw\\bin\\tk85"
```