ENH: Add show_fiff function

I used this function to debug an issue with reading/writing. It's basically an "enhanced" version of `mne_show_fiff()`. I figured it might be useful for others. Not extensively tested on anything other than forward solutions, but it's mostly for debugging anyway. Maybe I should make it `_show_fiff()` so users don't try to use it? (Or maybe actually add tests and try it on FIF files other than forward solutions?) Let me know what you think...

This is the type of output it produces:
```
>>> mne.fiff.show_fiff(fwd_fname, read_limit=None)
350 = FIFFB_MNE
    358 = FIFFB_MNE_ENV
        103 = FIFF_UNIT_PA/FIFF_BLOCK_ID (20)
        3550 = FIFF_MNE_ENV_WORKING_DIR (30) = /mnt/ssds2/larsoner/data
        3551 = FIFF_MNE_ENV_COMMAND_LINE (339) = mne_average_forward_solutions 
    353 = FIFFB_MNE_PARENT_MRI_FILE
        3508 = FIFF_MNE_FILE_NAME (62) = /mnt/ssds2/larsoner/data
        222 = FIFF_COORD_TRANS (104)
    354 = FIFFB_MNE_PARENT_MEAS_FILE
        3508 = FIFF_MNE_FILE_NAME (65) = /mnt/ssds2/larsoner/data
        222 = FIFF_COORD_TRANS (104)
        200 = FIFF_NCHAN (4) = [376]
        x375: 203 = FIFF_CH_INFO/FIFF_UNIT_AM_M2 (96)
        359 = FIFFB_MNE_BAD_CHANNELS
            3507 = FIFF_MNE_CH_NAME_LIST (169) = EEG 001:EEG 002:EEG 003:EEG 00
    351 = FIFFB_MNE_SOURCE_SPACE
        3518 = FIFF_MNE_SOURCE_SPACE_TYPE (4) = [1]
        3517 = FIFF_MNE_SOURCE_SPACE_ID (4) = [101]
        410 = FIFF_SUBJ_HIS_ID (4) = W001
        3506 = FIFF_MNE_COORD_FRAME (4) = [5]
        3512 = FIFF_MNE_SOURCE_SPACE_NPOINTS (4) = [137956]
        3510 = FIFF_MNE_SOURCE_SPACE_POINTS (1655484) = [-0.00915335 -0.06804086 -0.01 ... size=413868
        3511 = FIFF_MNE_SOURCE_SPACE_NORMALS (1655484) = [ 0.30601144 -0.93414593  0.18 ... size=413868
        3513 = FIFF_MNE_SOURCE_SPACE_SELECTION (551824) = [1 0 0 ..., 0 0 0] ... size=137956
        3514 = FIFF_MNE_SOURCE_SPACE_NUSE (4) = [3141]
        3590 = FIFF_MNE_SOURCE_SPACE_NTRI (4) = [275908]
        3591 = FIFF_MNE_SOURCE_SPACE_TRIANGLES (3310908) = [     1      2      7 ..., 137 ... size=827724
    351 = FIFFB_MNE_SOURCE_SPACE
        3518 = FIFF_MNE_SOURCE_SPACE_TYPE (4) = [1]
        3517 = FIFF_MNE_SOURCE_SPACE_ID (4) = [102]
        410 = FIFF_SUBJ_HIS_ID (4) = W001
        3506 = FIFF_MNE_COORD_FRAME (4) = [5]
        3512 = FIFF_MNE_SOURCE_SPACE_NPOINTS (4) = [136598]
        3510 = FIFF_MNE_SOURCE_SPACE_POINTS (1639188) = [ 0.01421047 -0.06941937 -0.01 ... size=409794
        3511 = FIFF_MNE_SOURCE_SPACE_NORMALS (1639188) = [ 0.18768917 -0.7944144   0.57 ... size=409794
        3513 = FIFF_MNE_SOURCE_SPACE_SELECTION (546392) = [0 0 0 ..., 0 0 0] ... size=136598
        3514 = FIFF_MNE_SOURCE_SPACE_NUSE (4) = [3154]
        3590 = FIFF_MNE_SOURCE_SPACE_NTRI (4) = [273192]
        3591 = FIFF_MNE_SOURCE_SPACE_TRIANGLES (3278316) = [     1      2      6 ..., 136 ... size=819576
    352 = FIFFB_MNE_FORWARD_SOLUTION
        3522 = FIFF_MNE_INCLUDED_METHODS (4) = [1]
        3506 = FIFF_MNE_COORD_FRAME (4) = [4]
        3521 = FIFF_MNE_SOURCE_ORIENTATION (4) = [2]
        3512 = FIFF_MNE_SOURCE_SPACE_NPOINTS (4) = [6295]
        200 = FIFF_NCHAN (4) = [306]
        357 = FIFFB_MNE_NAMED_MATRIX
            3504 = FIFF_MNE_NROW (4) = [18885]
            3505 = FIFF_MNE_NCOL (4) = [306]
            3503 = FIFF_MNE_COL_NAMES (2753) = MEG 0113:MEG 0112:MEG 0111:MEG
            3520 = FIFF_MNE_FORWARD_SOLUTION (23115252) = [ -6.83882608e-05  -8.88390496 ... size=5778810
    352 = FIFFB_MNE_FORWARD_SOLUTION
        3522 = FIFF_MNE_INCLUDED_METHODS (4) = [2]
        3506 = FIFF_MNE_COORD_FRAME (4) = [4]
        3521 = FIFF_MNE_SOURCE_ORIENTATION (4) = [2]
        3512 = FIFF_MNE_SOURCE_SPACE_NPOINTS (4) = [6295]
        200 = FIFF_NCHAN (4) = [70]
        357 = FIFFB_MNE_NAMED_MATRIX
            3504 = FIFF_MNE_NROW (4) = [18885]
            3505 = FIFF_MNE_NCOL (4) = [70]
            3503 = FIFF_MNE_COL_NAMES (559) = EEG 001:EEG 002:EEG 003:EEG 00
            3520 = FIFF_MNE_FORWARD_SOLUTION (5287812) = [ 41.65768814  55.14899063  57 ... size=1321950
```