List of planned pipes

There's a list of somes pipes that I found useful to add in this lib:

**Array**:

- [x] meanPipe - [**#7210b63**](https://github.com/a8m/ng-pipes/commit/7210b63ed937ed269b8f17d4f331b63688cda394)

**Math**:

- [x] ceilPipe - [**#a65a047**](https://github.com/a8m/ng-pipes/commit/a65a047f1b03b2aab6c7d0392d1c1d413c06987c)
- [x] floorPipe - [**#c4c7bae**](https://github.com/a8m/ng-pipes/commit/c4c7baec0f2b5ad7855eb23f5fdefac7d4bbdf52)
- [x] powPipe - [**#fad4200**](https://github.com/a8m/ng-pipes/commit/fad4200e75b71452a2b82da0e29a6f3cc8d8257f)
- [x] roundPipe - [**#20d47c6**](https://github.com/a8m/ng-pipes/commit/20d47c662f3423c5ce92fe61f6a8e29a049cf4a0)
- [x] sqrtPipe - [**#c62b6e6**](https://github.com/a8m/ng-pipes/commit/c62b6e6aa157471772b63fc02f9db77a30ae8070)

**Object**:

- [x] invert - [**#2547e10**](https://github.com/a8m/ng-pipes/commit/2547e108e051483a9d01aace23ad471f913cf550)
- [x] keysPipe - [**#73afe11**](https://github.com/a8m/ng-pipes/commit/73afe11a78b67739a8e1ed108740e5f091233b14)
- [ ] omitPipe
- [ ] pickPipe
- [x] valuesPipe - [**#e55f413**](https://github.com/a8m/ng-pipes/commit/e55f413e47e63d9f31e0de8a76ba12d8a29e2798)

**String**:

- [x] rightPadPipe [**#ef66e52**](https://github.com/a8m/ng-pipes/commit/ef66e52ff1d3ba3a1844962b3409e05468a5ba33)
- [x] leftPadPipe - [**#ff7c2ca**](https://github.com/a8m/ng-pipes/commit/ff7c2caea2388a3856cdf5be9b723efcdbc2a718)
- [x] centerPadPipe - [**#0d8c993**](https://github.com/a8m/ng-pipes/commit/0d8c99323ecb30900aff81453ce4c3a0dd393be3)

If someone want to suggest a pipe, please put here in comments.